Remove ENABLE_OVERVIEW_ACTIONS flag, defaulting to true

We no longer support the "peeking" model where the shelf would
peek on motion pause from an app and overview would peek on motion
pause from home. Thus, removed/inlined the following:
- FlingAndHoldTouchController (merged into its sole subclass
  NoButtonNavbarToOverviewTouchController)
- ShelfPeekAnim
- OverviewPeekState

Change-Id: I066a3ad2636fde4786089c922b896bf1e03361fd