Reducing tests flakiness.

Moving some tests to TaplTests. It's a right idea in general, also
flakes seem to go away because of a different initialization pattern.

Reintroducing UIDevice.pressHome in the test's start. Once BVTs pass,
I'll switch back.
See b/121335479

Test: All nexus tests
Change-Id: I677be5b6aaf073d15e8f2cf9b7a940a1f457b325