Don't unregister supplementary service notification.

Currently unregistering with RIL for supplementary service
notification have caused issues wrt to not showing notification.
On unregistering, RIL will also unregister for supplementary
service notification and the delay to register again can cause
telephony to miss the notification.

Fix: Register to RIL for supplementary service notification only
once when radio becomes available, instead of every time a new
connection is created.

Don't unregister for supplementary service notifications
from RIL after call is disconnected.

Change-Id: I7b911314bffad6980d7bfee0918133ae87f2ec09
CRs-Fixed: 2454098