Fix Telecom ConnectionTest.

A recent Telephony change was made to support missed calls reported
over SMS; this change made FailureSignalingConnection fully mutable
which failed some CTS.
Given that its only the setAddress method which would fail the mutability
check, re-added immutability in the FailureSignalingConnection since
ultimately we do want to protect these from state changes, but removed
the mutability check in the setAddress method since its reasonable to
want to report an address in this case.

Test: cts-tradefed run cts-dev -m CtsTelecomTestCases -t android.telecom.cts.ConnectionTest
Fixes: 150811816
Change-Id: I7f37daf9ac65ff70eec7692410fa299cb30fdc38