Proper mouse support for Android backend

This PR adds the handling of mouse cursor movement and scroll wheel on Android 3.1+. It also correctly report the mid and right mouse buttons on 4.0+

I've tested this with a USB mouse and the touchpad on my ASUS Transformer tablet. (Had to manually configure the tablet not to report mid/right buttons as menu and back.) I've also tested it on my old Archos 43 to make sure I didn't break anything all the way back to Android 2.2.