Use ServiceState to get data cell ID

ServiceState now contains a CellIdentity that can be obtained
specifically from the data subscription, and for PS on cell
only.

This should be used for connectivity level network monitoring
info.

Bug: 123893112
Test: 1. atest NetworkMonitorTest
      2. Verify if cell id is correct

Change-Id: Ieb00de84a0d518e3197e3884f946b872e998304a