Introduce BackupManagerServiceInteface in remaining parts of the code.

Use it in Trampoline, KeyValueAdbBackupEngine, KeyValueAdbRestoreEngine
where it wasn't used automatically, because of using package-private
methods and fields. To be able to do this, also make couple of methods
public and move them to the interface and add getBackupManagerBinder()
method to use instead of directly accessing the field.

Test: not required as behaviour is not modified.

Bug: 36850431

Change-Id: Ia7a26f93f8dba37ece87305979c922acb58a271f