Show x for short period of time when activity is PIP'd

Show x for short period of time when activity is PIP'd

- Moves dismiss view out of menu container layout and manages the alpha
  for it separately (i.e. when animating menu in / out or modifying the
  alpha when dragging to dismiss)
- Shows the dismiss view when PIP mode is entered, fades out after
  menu time out, on interaction the timeout is reset

Test: Enter PIP, note 'x' that is displayed, tap on it to dismiss the PIP
      or don't tap on it and it goes away
Bug: 36900043
Change-Id: Ia596ee60cd9c0d744e459564586ba6a0a9c23d5d