Makes AccessibilityEventTest and homeScreenOrBust more robust.

- Uses launchActivityAndWaitForItToBeOnscreen() in
  AccessibilityEventTest.
- Injects KEYCODE_BACK when opening the home screen to hide
  any non-system dialogs.

Bug: 263942937
Bug: 265852111
Test: AccessibilityEventTest running within the test suite
      v2/android-xts/cts/framework/device-release
Change-Id: If3b48b8ff2cf19f993ab2d4846b017aa8b150760
Merged-In: I40211679e4dfc19b65bb41da78283c91ab30cb2c
5 files changed