Upgrade to Android Gradle plugin v0.9

Move src/instrumentTest to src/androidTest.
Upgrade build tools to 19.0.3
Upgrade Gradle to v1.11

See also [Migrating From 0.8 to 0.9](http://tools.android.com/tech-docs/new-build-system/migrating_to_09).

/cc @raymondfeng FYI.