Added Cell identity for TD-SCDMA and a base class CellIdentity

Added CellIdentityTdscdma for TDS-CDMA cell identity. Also added
a base class for existing CellIdentityXxx so that the network
service can return specific RAT cell identity through the
network service callback API which takes generic cell identity
as the parameter.

This is the preliminary change for IWLAN refactoring since cell
identity is part of the CS/PS network registration.

bug: 64132030
Test: Unit tests
Change-Id: Ieea70a9e4d05ff2710c9d11de2f0984996817f73