Adding *.jar to the Project is done by using build.gradle file:
- If you want to add a jarfile to the project, Facebook had already done on your behalf!
 Just add alibsfolder into theandroid/appdirectory of the project with yourjarfile and enjoy!
 

- If you want to add a jarfile to anative-modulethen add the line ofcompile fileTree(dir: "libs", include: ["*.jar"])into thedependenciespart ofbuild.gradlefile of the native-module.
Example-1:
After I added okhttp-3.4.1.jar file into the lib folder, I also add that package name to the dependencies part as the following:
dependencies {
    compile fileTree(dir: "libs", include: ["*.jar"])
    compile 'com.facebook.react:react-native:0.19.+'
}
Example-2:
If I need another package -that is found in Maven repo- I have to add into dependencies block as following (for instance I wanna add fresco):
dependencies {
    compile 'com.facebook.fresco:fresco:1.9.0'
}
Then Gradle will find and install dependency library Fresco for me.
Usually, every Android project has already Maven repo. configurations in build.gradle file which is found in top of folder of the project.
For example:
allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com"
        }
    }
}
Example-3:
(I have never tried this however it should be worked) 
If I have an drawee-debug.aar file I can add it into my project by putting it into lib folder as directed on Example-1 then I have to change fileTree line as following:
compile fileTree(dir: "libs", include: ["*.jar", "*.aar"])  // "*.aar" is added
Example-4:
(alternate way of Example-3) 
If I have an drawee-debug.aar file also I can add it into my project by putting it into lib folder as directed on Example-1 then I have to change and add some lines as following:
dependencies {
    compile (name:'drawee-debug', ext:'aar')
}
allprojects {
    repositories {
        ...
        flatDir {
            dirs 'libs', './libs'
        }
        ...
    }
}
In this way, libs directory is defined in allprojects folder and aar file specified in dependencies block, like othe examples. 
Note that after Gradle v3.0.1 implementation is used instead compile key word.
Kudos: https://stackoverflow.com/a/37092426/3765109