Fixing issue with screen pinning screen not showing for secondary users.

- Catch the exception when removing a view that is not yet attached. 
  This is not an issue because we are in the process of attaching it for
  the first time anyways.
- Ensure that we post the calls from the secondary user in the main 
  thread for the system user.  Especially since this calls through to
  add the view for the new screen pinning window.

Bug: 29341417
Change-Id: If0e29a77ddb67426115f1d5d9f1adb580a1f07df