support data migration from OEM legacy app to mainline

Defined systemAPI for interaction with legacy cellbroadcast app

Bug: 135956699
Test: Tested with cell broadcast receiver test app and confirmed data can be migrated.
Merged-In: I6e583672a3673fd226e14d33d7a7648f4fd03efb
Change-Id: I6e583672a3673fd226e14d33d7a7648f4fd03efb
(cherry picked from commit ad2f91fc)