PhoneWindowManager: Only react to camera key when necessary

* With the camera already open, long pressing the camera key opens
  another instance of the cam app in front
* each of the newly started instances get added to the stack
* no need to pause the current one and bring another one to the front
  when we are already at the desired state

Change-Id: Ieaf50cc9bbec92da19787928b48d1f3806efb6b4