[Multi-User] Make JobScheduler logic multi-user aware

Patchset 1: Introduce BackupJobIdManager to allocate/recycle IDs for KeyValueBackupJob and
FullBackupJob

Patchset 2: Update KeyValueBackupJob and FullBackupJob to work for multiple users — take in
userId as a parameter and use BackupJobIdManager to assign job IDs

Patchset 3: Update robo tests

The corresponding GTS ScheduledBackupHostSideTest will be updated in a separate CL with the same
topic.

Bug: 121198009
Test: 1) atest KeyValueBackupJobTest
      2) atest KeyValueBackupTaskTest
Change-Id: Ifdba254b7084b3d0a1507cc6512e98399c1aad8f