Fixes for a few bugs related to visible-behind activities

   - Allow activities currently not visible to switch back to not
     visible-behind state.
   - Do not allow an activity that is not set as the visible-behind
     activity to cancel the visible-behind state.
   - Immediately mark a finishing activity as no longer visible behind.

Bug: 17070266
Change-Id: I2c5016793e5264013b8a54b84098e6ffae34ae6c