Some overview scrim and status bar color adjustments

- Replace UI_STATE_OVERVIEW with UI_STATE_FULLSCREEN_TASK. Overview no
  longer needs it's own status bar treatment as it's handled by
  ScrimView already. So remove instances of UI_STATE_OVERVIEW and
  replace some with UI_STATE_FULLSCREEN_TASK if a TaskView is behind
  the status bar.
- Add ScrimView to fallback recents activity.
- Remove scrim from Background state, don't fade it in until the
  gesture ends to RECENTS.

Test: watch scrim and status bar colors during the following
- Swipe up to home (no scrim, status bar matches task at first but
  then wallpaper once task isn't overlapping status bar)
- Swipe up to overview (no scrim until let go to overview, status
  bar matches task at first, then wallpaper after not overlapping,
  then scrim once it's 90% opacity)
- Quick switch from home (scrim fades in, status bar matches once
  it reaches 90% opacity)
- Quick switch from app (no scrim, status bar matches center task)
- Repeat above for 3P launcher and 2 button mode

Fixes: 185681676
Fixes: 185286870
Change-Id: I9e5c292cc734e714169b9cc8268e8dad4e868aca