[Multi-user] Fix BackupManagerServiceTest checkstyle

Similar to ag/5541540, fix the BMS test for the checkstyle preupload
hook since it will complain when we migrate BMS -> UBMS.

This does not change any test functionality, only adds javadocs.

Bug: 118520567
Test: atest BackupManagerServiceTest
Change-Id: I71800b13f2ef45d3533bedd9878761b0e6c61f0f