Added area info support in cell broadcast service

Added a broadcast for area info changed event, and
also a get method in cell broadcast service to
retrieve the area info.

Test: Cell broadcast sanity tests
Bug: 141318859

Merged-In: I5995e3c5099a1e49cb0c54d33da1dbfcc94d4037
Change-Id: I5995e3c5099a1e49cb0c54d33da1dbfcc94d4037
(cherry picked from commit afaa71d4)