Migrate from Gradle to Maven.

- Restructure source to src/{main,test} style
- Add pom.xml and update Android.mk
- Migrate all tests to JUnit4 and Robolectric
 - RequestQueueTest is currently @Ignored as fixing it will
   involve more extensive refactoring.
- Main library still builds in Gradle; tests do not

Change-Id: I1edc53bb1a54f64d3e806e4572901295ef63e2ca
Merge "Migrate from Gradle to Maven."
Merge master change