Setup codepath to animate the Taskbar when going to and from Launcher,
primarily by listening for pause/resume signals but also hints from
gesture nav and AppToOverviewAnimationProvider.
Additionally, add TaskbarStateHandler to listen for Launcher state
changes if Taskbar is enabled. Combined, the end behavior is:
- Background alpha is 0 when Launcher is resumed, and 1 when Launcher
is paused (we can make this animation more interesting later).
- Taskbar is always visible when Launcher is paused, otherwise its
visibility is determined by multiple factors: LauncherState and
whether the IME is showing.
Bug: 171917176
Change-Id: I7856fc979931c9d12d714dee11d179fd1b5a6968
