Get rid of reflection in Android backend 

Since Libgdx minimal Android version is 2.2, we can get rid of old reflection techniques and some unnecessary checks. No more reflection in Android backend except `AndroidLiveWallpaper.java` (comments in this class are misleading in some way, so I decided to leave it alone). Also, `AndroidSingleTouchListener.java` is unused anymore but I didn't dare to remove it.