Properly notify of precise call state change after connection hangup.

When disconnecting connections via Connection.hangup(), previously no
precise call state notification was sent out.
This change should fix BT headsets ringing indefinitely after a
blacklisted call came in, as BluetoothPhoneService can now update the
headset state properly.

Change-Id: I4439d7d8ed47e72da9d061b885ec9a0e6e1b784d
Telephony: prevent NPE

Change-Id: Ia7a08334a901ef67353d5816d0822d0cb2d57daf