java: Release 2.1.5

This has already been uploaded to bintray and maven.

Notable changes:
- JNI reference errors are gone
- sync watcher doesn't poll any more

The TODOs app works with the latest changes, though there is still
some bluetooth-related flakiness.

Change-Id: I77c0c036c2ef8bbddbe52efe95c57454614b7581
2 files changed