Refacotr mobile data observer for repository

In current design, the mobile data observer might be removed due to
SIM absence after registered so that mobile data network info can't
be updated to clients.

To fix it, migrate it to use API TelephonyCallback.onUserMobileDataStateChanged

Bug: 327696232
Test: manual + unit tests

Change-Id: I26c8f946823abb7505f0227c8dd8ab0700d0c5f2
Merged-In: I26c8f946823abb7505f0227c8dd8ab0700d0c5f2
2 files changed