Fix getLevel for CellSignalStrengthTdscdma

For backwards compatibility (and because it's the
correct choice anyway), getLevel() should
use RSCP as the default signal strength calculation.

-Remove the unused RSSI level constants
-Add RSCP level constants and update the calculation.

Bug: 128369744
Test: atest CellSignalStrengthTdscdmaTest
Change-Id: Ia388a872ee6a0c02b74046aef1f04b0b4cf59130