Disallow multiple calls to dismiss/exit pip

- When exitPip or removePip is called we don't set mInPip=false
  until the actual changes are commited (either animation is started
  or task actually vanishes) so if there are multiple subsequent calls,
  it could fall into the same code twice (haven't actually been able
  to reproduce the bug but this is one case it could happen)

Change-Id: Ica2836827114fdd5affb983ab4444e9eb0c00283
Bug: 157532238
Test: Not actually able to repro