Extract interface of BackupManagerService and make BackupManagerService implement this interface.

This is part of (mostly) automated refactoring of BackupManagerService, see go/br-bm-automated-refactor.

All done automatically with https://www.jetbrains.com/help/idea/2016.3/extract-interface.html

Test: not required, as this doesn't change any behaviour.

Bug: 36850431

Change-Id: I1efa15b0f600536a9fd555bed18f2e86c3bcfecb