Fix NullPointerException in VideoCallImpl.

There were instances of NPEs due to a null mHandler in the VideoCallImpl.
Traced the problem down to InCallService implementations which do not
register a VideoCall Callback (e.g. gearhead).

Added null checks for uses of mHandler.

Bug: 21474154
Change-Id: I44dffde5e46529cc62912d417ad23c7b7dd1a1ba