Cleanup taskbar+assistant interactions

- Always move the taskbar layer when voice interaction window visibility
  changes, to ensure we remove the temporary taskbar background in all
  cases (e.g. swipe to home)
- Move check to not draw taskbar background to the temporary view
  itself, so that it's at the proper layer when shown (e.g. during
  fulfillment on the home screen)
- Don't add view to window if it's already added (we already have this
  check for the removal case)

Test: Open an app, invoke Assistant, swipe home; Taskbar background is
removed; invoke and fulfill assistant from home screen, in app, in 3
button mode, and in gesture mode (all permutations)
Fixes: 236924161
Fixes: 240101132
Fixes: 235895444

Change-Id: I5958f1dbe14a6d190d3808014429f7f8146c9f96