Gradle Cleanup

DO NOT MERGE YET (I'll push recent subproject commit refs to this PR as soon as https://github.com/DroidPlanner/usb-serial-for-android/pull/2 and https://github.com/DroidPlanner/HorizontalVariableListView/pull/3 are merged)

Follow up to https://github.com/DroidPlanner/droidplanner/pull/619

* Removed *.iml from version control as they contain too much local data and thus changing arbitrarily
* Ignoring sub project 'google-play-services_lib' and added them as Maven dependencies
* Mavlink/Google Play Services: API updated (forgot that in previous PR)

With this PR, the project can be imported in Android Studio hassle-free:
* Prepare a clean clone with updated submodules 
* Import project in Android Studio
* Sync from Gradle Files
* Build & Run