Avoid layout changes on call disconnection.

This avoids showing the call buttons when an incoming call is
disconnected remotely while still ringing, which in turn avoids an ugly
transition when using the full screen caller photo.

Change-Id: If69bce271303ea43dca64925aba895dd3b5b5611

Conflicts:
	src/com/android/incallui/CallButtonPresenter.java
	src/com/android/incallui/InCallActivity.java