For anyone looking for the answer, hope this helps: In order for us to use any Google Android library (or 3rd party Android library) in our ANE, you have to find out the real version number of your Android library then insert that number into your manifest file. For example, instead of
<meta-data android:name="com.google.android.gms.version"
android:value="@integer/google_play_services_version" />
We have to find out the real version number and your manifest data should be :
<meta-data android:name="com.google.android.gms.version"
android:value="4432145" />
In this case, the number 4432145 is the version number of the GooglePlayService library that we want to use. You can find out this number in the Resource of the Android library project. In the future if you want to upgrade to use the latest version of GooglePlayService library, you have to rebuild your ANE file and correct the version number to the new version number of the library in your manifest file.