Hide taskbar while VoiceInteractionWindow is visible

- Fade out icons (or stashed handle if stashed)
- Move background to APPLICATION_OVERLAY layer (behind VIW)
  - Only for gesture nav
- On home screen in 3 button mode, translate buttons down when assistant
  is invoked

Test: Invoke assistant, ensure it draws above taskbar background and
taskbar stops drawing icons or stashed handle
Bug: 225200928

Change-Id: I7807d30df8f358295b8231d89217c7413f32c6f9
Merged-In: I7807d30df8f358295b8231d89217c7413f32c6f9