Fix CellSignalStrengthNr to use SS-RSRP

SS-RSRP will be returned as the default implementation
of getDbm(). Becuase it's based on a broadcast signal rather
than a UE-specific signal, it's more universal, and it also
is consistent with other measure of SignalStrength.

Bug: 143171541
Test: atest CellSignalStrengthNrTest#testGetMethod
Change-Id: Ib4312c28629f2a22497b0292afcff38d9e2e08b6