Fix BackupManager.isBackupEnabled()

It wasn't properly lazy-initializing the service binder, so it always
thought the backend service didn't exist, and so always returned false.

Also directly validated that every usage of sService in the module is
now correctly lazy-initialized.

Bug 16661321

Change-Id: If5fbb18aef81bfa8fd70eb40a1f6af54cc96d804