Introduce getRssi() in CellSignalStrengthLte

Previous people designed that the lte signal strength from HAL is rssi
in ASU, but the new need of getRssi() API requires dBm unit.

Bug: 113074174
Test: Treehugger
Change-Id: I2982365a2e7fe1a56a393d123b9b0a30807d4d15