Refactor SignalStrength

Refactor SignalStrength as a container of
individual CellSignalStrength objects for each
radio technology.

-Change SignalStrength to contain CellSignalStrength
 rather than individual measurements.
-Change the constructors for SignalStrength to take
 CellSignalStrength objects.
-Consolidate all the logic for calculating the getLevel()
 function to CellSignalStrength.
-Store the level value in SignalStrengths rather than
 compute it on the fly when needed.
-Add a few missing functions needed by SignalStrength:
     -CellSignalStrengthCdma#getEvdoAsuLevel()
     -CellSignalStrengthTdscdma#getRscp()
-Convert CellSignalStrength classes to take actual values
 instead of ASU values.

Bug: 25984037
Test: atest SignalStrengthTest
Change-Id: I302e7a8c6f68559dc69a3138061eec739ee7561f