Add test for long-press back

Adds a test to ensure:
  - long-pressing the back button on non-watch devices always
    delivers a down and up event to the foreground app
  - the foreground app does not get dismissed before the up event

BUG: 27533831
Change-Id: I69ab2d3587a5865fcb7ce30f7aa2be3a8878031b
3 files changed