Install android targets and android sample app

This app is self-contained and can be compiled and deployed on devices (not by pants yet, of course).

The icon is from the Android SDK, which is released under the Apache license. I can see a world where 
a Pants icon would be preferred. I am comfortable putting the Pants license on everything else as I 
mostly wrote it by hand.

The targets have all been moved in backend.android and include android_target, android_binary and
android_resources.