Fix NullPointerException in SignalStrengthController

The CellIdentity parameter of method updateServiceStateArfcnRsrpBoost
comes from the calling of NetworkRegistrationInfo#getCellIdentity
which can be null without ACCESS_FINE_LOCATION permission.

The CL also throughly scan all the methods and make sure
the variables and parameters are properly annotated with
@Nullable or @NonNull and perform null check for all
@Nullable before usage.

Bug: 218749349
Test: atest SignalStrengthControllerTest
Change-Id: Iefc3c72bb31294243ac5276d00631609c9de455a