Make incoming call screen style configurable (2/3)

Change-Id: I46db78c9b48fae52530b96789ec2721d51b7185a
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