Pass operation type via BackupManagerMonitor#onEvent

Events sent via BackupManagerMonitor don't contain the operation type
they're associated with (backup / restore) making them useless for
logging and debugging in some cases.

Bug: 279941227
Test: atest FrameworksMockingServicesTest
Change-Id: I8450a2404a0e4a0c4d82b62980cda1e5639f424f