Synchronize onCallAdded with onCallRemoved.

In cases of very quick call and hangup we can sometimes get call
disconnected (and onCallRemoved) status chagned before we get
onCallAdded. This is because onCallAdded currently does not hold the
lock.

Bug: b/28969591
Change-Id: Ic2f291dff9dd61c54b1ada3b21c61e24bca65ebc