Switched TelephonyBackupAgent to ContentResolver

It used instances of content providers (SMS, MMS). Since now it's loaded
not in the restricted state it could safely use ContentResolver.

Also took all the functions out of the static scope into the object scope.

Bug: 27791017

Change-Id: I468bb4859bddb0fba1db388101bd1ddecfc3ce0c
2 files changed