Fixes to VoiceInteractionWindowController

- Make temporary taskbar background behind assistant non-touchable
  (fail-safe in case the window isn't removed for some reason)
- Give temporary taskbar background a different window title and
  add a couple more dump statements to help debugging
- Only show the taskbar background for persistent taskbar; transient
  taskbar can skip most of the special casing and just hide it
- Fix bug where we weren't drawing the separate taskbar background in 3
  button mode
- Fix bug where we weren't actually synchronizing
  separateWindowForTaskbarBackground with TaskbarDragLayer, since the
  former wasn't attached to the window yet; now we wait until it is
  attached before calling synchronizeNextDraw()
- Also added dump logs for TaskbarDragLayerController alpha channels

Test: manual in 3 button and gesture nav (with and without
FORCE_PERSISTENT_TASKBAR enabled)
Fixes: 243652789
Bug: 262664266

Change-Id: I865871e57dd4cb255a916317a7e5d35cfde97df5