Encapsulate RefactoredBackupManagerService fields.

This encapsulates back the fields which were decapsulated when splitting
out internal classes.

Bug: 37520921
Test: adb shell settings put global backup_refactored_service_disabled 0
&& adb reboot && adb shell bmgr backupnow --all
Change-Id: I9caa75b2f688de96bd5b245f43e0ae66cd9d023c