Add AndroidTarget and AndroidBinary to pants

Several of the fields are passed default values, which will later be
either defined elsewhere or will be filled by the future
AndroidManifest parser (which will fill fields like target_sdk and package).

AndroidBinary has little difference from AndroidTarget, but was created
with future support of AndroidLibrary in mind.

Testing Done:
./pants goal test tests/python/pants_test:all and ci.sh both passed.

Adding pants.backend.android to base/dev_backend_loader causes tests to fail, so that has been left out of this patch for now.

Reviewed at https://rbcommons.com/s/twitter/r/572/