Don't add multiple instances of CircularRevealFragment

Certain calls can propagate state changes immediately after
the call is made, causing InCallActivity.internalResolveIntent
to be called twice in succession.

If multiple animations are run the UI is left in an inconsistent
state.

Bug: 19842284
Change-Id: Ie6060a37d2bd7775731e0e82019bb8baf0ffd834