Access Identity and SignalStrength from CellInfo

Accessing common CellIdentity and CellSignalStrength
information should not require RTTI. Thus, add
overloaded methods that allow the access the common
versions of these functions from a top level CellInfo.

Bug: 10151416
Test: compilation

Change-Id: I18f31c5cec075dcd68cc493e24c2d551138dd73c