I updated the Android Studio to version 3.1 and the Gradle for my Project to version 3.1.0. Since then I had some problems related to merging multiple files, but that I was able to solve. Then I had this error:
> Manifest merger failed : 
Attribute activity#com.aware.ui.PermissionsHandler@launchMode 
value=(singleTop) from [com.awareframework:aware-core:4.0.555] 
AndroidManifest.xml:35:13-43 is also present at 
[com.github.denzilferreira:aware-client:4.0.555] 
AndroidManifest.xml:44:13-42 value=(standard).
Suggestion: add 'tools:replace="android:launchMode"' to <activity> 
element at AndroidManifest.xml:30:9-37:66 to override.
So, first I tried to use tools:replace="android:launchMode" inside the mentioned Activity:
<activity android:name=".MainActivity"
    ...
    tools:replace="android:launchMode"
    android:launchMode="standard">
</activity>
But it did not solve the problem, then I searched and found some similar questions and their responses.
One of them said to delete the proposed attribute from the libraries that were conflicting:
<activity
    android:name="com.aware.ui.PermissionsHandler"
    android:configChanges="keyboardHidden|orientation|screenSize"
    android:excludeFromRecents="true"
    android:exported="true"
    android:launchMode="singleTop" //deleted this line from Manifests
    android:noHistory="true"
    android:theme="@style/Theme.AppCompat.Translucent" />
But again, with no success. I also tried to use tools:replace="android:launchMode" inside the application tag:
<application
    ...
    tools:replace="android:launchMode">
An then use the android:launchMode="standard" inside the activity tag.
<activity android:name=".MainActivity"
    ....
    android:launchMode="standard">
</activity>
But it throws a different error:
tools:replace specified at line:16 for attribute android:launchMode, 
but no new value specified
I also tried to reorder the dependencies inside the Gradle files, like @GLee answered here but it did not make any difference.
This is one of the libraries I'm using that is conflicting: Aware Activity Recognition Plugin.
And this is the tutorial I used to create the application: Creating a standalone application.
Another relevant thing to say is that the two dependencies conflicting are in different modules, the com.awareframework:aware-core:4.0.555 dependency is inside the app module and the com.github.denzilferreira:aware-client:4.0.555 is inside the activity_recognition module.
Finally, this is my App Gradle file:
apply plugin: 'com.android.application'
android {
    compileSdkVersion 27
    buildToolsVersion "27.0.3"
    defaultConfig {
        applicationId "app.miti.com.iot_reduce_daily_stress_application"
        minSdkVersion 19
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        multiDexEnabled true
    }
    ...
}
dependencies {
    ...
    implementation "com.awareframework:aware-core:${aware_libs}"
    implementation "com.android.support:appcompat-v7:${support_libs}"
    implementation "com.android.support:cardview-v7:${support_libs}"
    implementation "com.google.android.gms:play-services-location:${gms_libs}"
    implementation "com.google.android.gms:play-services-maps:${gms_libs}"
    implementation "com.google.android.gms:play-services-places:${gms_libs}"
    implementation 'com.android.support:preference-v14:${support_libs}'
    implementation "com.android.support:design:${support_libs}"
    implementation 'pub.devrel:easypermissions:1.2.0'
    implementation 'com.android.support:multidex:1.0.3'
    implementation "com.google.firebase:firebase-core:${firebase_libs}"
    implementation "com.google.firebase:firebase-messaging:${firebase_libs}"
    ...
}
And this is my activity_recognition module Gradle file:
apply plugin: 'com.android.library'
android {
    compileSdkVersion 27
    buildToolsVersion '27.0.3'
    defaultConfig {
        minSdkVersion 19
        targetSdkVersion 27
        versionCode version_code
        versionName version_readable
        multiDexEnabled true
    }
    ...
}
dependencies {
    ...
    implementation "com.google.android.gms:play-services-location:${google_libs}"
    implementation "com.android.support:appcompat-v7:${support_libs}"
    api "com.github.denzilferreira:aware-client:${aware_libs}"
    implementation "com.koushikdutta.ion:ion:2.1.6"
    implementation "org.eclipse.paho:org.eclipse.paho.client.mqttv3:1.0.2"
}
This is the App AndroidManifest.xml (error URL directs to this file):
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="app.miti.com.iot_reduce_daily_stress_application">
    ... //permissions
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        android:name="android.support.multidex.MultiDexApplication">
        <activity android:name=".MainActivity"
            android:configChanges="keyboardHidden|orientation|screenSize">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".SettingsActivity"
            android:configChanges="keyboardHidden|orientation|screenSize"
            android:parentActivityName=".MainActivity"
            tools:ignore="UnusedAttribute">
        </activity>
    </application>
</manifest>
Since this is not a dependencies version problem, like many other questions I found, and because I have to use this library dependencies in my Project, how can I solve this problem?
P.S: I have already tried the usual methods:
- Invalidate Caches/Restart
 - Deleted Gradle
 - Clean Project and Rebuild Project
 - ...