Add atoms for CellBroadcastModule

This topic adds device-side changes for local development. For
modulefooding, additional server side changes will be needed.

Bug: 147353892
Bug: 147244286
Test: manually trigger metrics and verify that they are sent with
statsd_testdrive
Change-Id: Ideb9033cd64b49e6489baf9748736cc4d2529b59