Don't hide end call button until call is disconnected

Currently, the end call button is hidden the moment the user
initates a hangup. In the event that the call is not actually
disconnected, the UI is stuck in a "Hanging up" state that is
unresponsive

For the majority of calls that disconnect correctly, there should
be no user-perceptible difference in behavior. For calls that are
not disconnected correctly, the end call button will remain showing
so that hangup commands can continue to be sent that will eventually
disconnect the call correctly.

Bug: 19933863
Change-Id: I2ff2018d7d229615f5f57c599f74954ec7492f6b