Update build.gradle to work with Android plugin v0.12.0
Idling resources for testing with Espresso

Here we wrap concrete `Downloader` and `GalleryService` so that when network call is made, these components would register themselves as non-idle, and notify listeners (i.e. Espresso) then they became idle once network request completes.
Some useful utilities for integration tests
Basic Espresso test with screenshots via Spoon