remove unused private fields and unnecessary TODOs

1. the private members in BackupManagerServiceTest were all unused
2. remove TODO for b/124359804 which is marked as wont-fix.
3. remove adb backup multi-user TODOs as adb backup is deprecated and we
don't intend to extend multi-user support to it.

Bug: 136738613

Test: m -j
Change-Id: Ia4c6ebce7b9bb7ab8bfe1f6a18f9b67ec6098cd2