Add espresso UI tests for the example app

[miguelandres]

Espresso UI tests are basically integration tests. Our coverage is currently very good on unit tests for the widgets and core, but not so much on the rest of the UI.

Espresso tests are particularly hard to do on recyclerviews like the RemixerFragment, furthermore some of our widgets (even basic ones like a seekbar) don't have native Espresso ViewActions so we may have to roll our own.