Do not register event until entering the state

Register the event when entering the state, and unregister when
exiting the state to avoid the event coming unexpectly due to
the timing issue among different threads.

Bug: 270912888
Bug: 271009884
Test: manually to kill phone process and check no crash
Test: atest com.android.internal.telephony.CellBroadcastConfigTrackerTest
Change-Id: I56545895dc5a1ddefce8df607f6d53bfe4696833