Start BackupManagerConstants content observer

BackupManagerConstants has a content observer that is meant to be
notified whenever backup settings change. However, that observer is
never registered.

Bug: 71622864
Test: cts-tradefed run cts-dev --module CtsBackupHostTestCase
Change-Id: Icbd90ef3af83dd2f29e26a5d787505bcf5681154
2 files changed