This patch adds an initial set of UI tests for Aegis built using Espresso. It
covers a fair bit of the essential functionality of the app, but there are lots
more tests we could add later on.
This also reconfigures our Travis CI build manifest to run the tests on API 21,
23, 27 and 28 emulators. It was a real pain to get this to work well, but let's
hope it's stable now.
I had to downgrade ``com.google.android.material`` to 1.0.0, because 1.1.0
introduced an issue where the test would hang.
Alexander Bakker