Two-zone model: swipe up from nav bar vs above it

When ENABLE_OVERVIEW_ACTIONS flag is enabled, swiping up from the nav
bar goes to overview if you hold, or the first home screen if you don't.

- Added NoButtonNavBarToOverviewTouchController, which extends
  FlingAndHoldTouchController but only works if you start from the nav
  bar. Otherwise it falls back to PortraitStatesTouchController to
  handle normal state transition to all apps.
- Added HintState. This is where the workspace/hotseat/qsb scale down
  when you swipe up from the nav bar, to hint that you're about to
  either go to overview or the first home screen.
  - Added getQsbScaleAndTranslation() to allow Overview and Hint states
    to treat it as part of the hotseat.
  - Since Overview needs to show above the QSB as it's animating, bring
    Overview to the front and update OverviewScrim to handle the case
    where there's no view above Overview to draw the scrim beneath.
- ENABLE_OVERVIEW_ACTIONS is always false in 2-button mode, since the
  shelf is crucial to that mode.

Bug: 143361609
Change-Id: I743481bb239dc77f7024dc98ba68a43534da2637