[Multi-user] Clean up backup enabled initialization

Remove indirection in setting backup enabled state to call directly into
the UserBMS instance.

Bug: 121305979
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) During SUW -> backup initializes off;
Turn backup on in SUW -> backup enabled;
Subsequent boots -> backup initializes on
Change-Id: Ief688c4e1c14b443db87eda43ec0ee398282662f