I have a code from this answer code in unity to install an apk but it does not work in Android 7.0 because Uri.fromfile is no longer supported and FileProvider.getUriForFile should now be used.
I tried opening the project in android studio and followed this tutorial to manipulate the manifest file - https://inthecheesefactory.com/blog/how-to-share-access-to-file-with-fileprovider-on-android-nougat/en
        AndroidJavaClass intentObj = new 
        AndroidJavaClass("android.content.Intent");
        string ACTION_VIEW = intentObj.GetStatic<string>("ACTION_VIEW");
        int FLAG_ACTIVITY_NEW_TASK = intentObj.GetStatic<int>
        ("FLAG_ACTIVITY_NEW_TASK");
        AndroidJavaObject intent = new 
        AndroidJavaObject("android.content.Intent", ACTION_VIEW);
        AndroidJavaObject fileObj = new AndroidJavaObject("java.io.File", 
        apkPath);
        AndroidJavaClass uriObj = new AndroidJavaClass("android.net.Uri");
        AndroidJavaObject uri = uriObj.CallStatic<AndroidJavaObject>
        ("fromFile", fileObj);
        intent.Call<AndroidJavaObject>("setDataAndType", uri, 
        "application/vnd.android.package-archive");
        intent.Call<AndroidJavaObject>("addFlags", FLAG_ACTIVITY_NEW_TASK);
        intent.Call<AndroidJavaObject>("setClassName", 
        "com.android.packageinstaller", 
        "com.android.packageinstaller.PackageInstallerActivity");
        AndroidJavaClass unityPlayer = new 
        AndroidJavaClass("com.unity3d.player.UnityPlayer");
        AndroidJavaObject currentActivity = 
        unityPlayer.GetStatic<AndroidJavaObject>("currentActivity");
        currentActivity.Call("startActivity", intent);