Expose RSSI as GSM RAT-specific Signal Strength

In GSM, RSSI is the correct specific name for signal
strength measurements. To promote callers using specific
methods and to bring GSM in line with other FooSignalStrength
methods.

Bug: 125027693
Test: cts - CellInfoTest
Change-Id: Idf62c7fbf8d6dd1c116da35a5e6375851218053e