[pytorch] simplify android circleci definition data model
Summary:
Android jobs don't seem to fit to `pytorch_build_data.py` data model
very well. Other mobile jobs all have their own data model files - even
for Android nightly jobs. As we are adding more variants like vulkan,
it's going to be hard to maintain.
So renamed `android_gradle.py` to `android_definitions.py` and moved
android jobs into it, following the conventions of `nightly_android.py`
and `ios_definitions.py`.
Differential Revision: D22036915
Test Plan: Imported from OSS
Reviewed By: IvanKobzarev
Pulled By: ljk53
fbshipit-source-id: 42ad5cbe451edecef17f6d3cbf85076cc3acf615
