Clear up ALWAYS_REPORTED_SIGNAL_STRENGTH_CHANGED logic

The feature is now replaced with TM#setSignalStrengthUpdateRequest
introduced in S release. The logic for the obsoleted feature is
cleared up.

The request to report system signal strength while idle is
still protected by permission to use internally.

Bug: 177924721
Test: atest TelephonyRegistryTest SignalStrengthUpdateRequestTest
Change-Id: Idad4ca59720fac50b5ec0fe50e2b6ac8e9dbb801