Register broadcastReceiver listening to ECM changes only once.

Since phone object is not disposed now (after phone switch refactor),
CallTracker needs to register receiver only once and not deregister/
re-register.
This is needed to fix a crash seen on trying to deregister an
unregistered receiver.

Bug: 25871221
Change-Id: Id057030f827a9cfd44584ef095a1297f6f8c1fae