[Multi-user] Disable backup by default in non-system users

Key changes in this CL:
- Backup is now disabled by default in non-system users unless DPM
activates backup for this user AND the system user is activated. This
provides gating for the multi-user B&R feature.
- Activation is done via an 'activate' file that is per-user (but lives
in the system user directory to account for locked users).
- isBackupServiceActive() handles both locked and unlocked users.
- Added a bmgr command to expose isBackupServiceActive() for testing
purposes and enforce appropriate permissions.

Future CLs:
- Handle future migration to backup on by default for non-system users
- Change CTS tests to use the new bmgr command

Bug: 121306407
Test: 1) atest TrampolineTest
2) Start system user -> service started; run backup and restore
successfully
3) Start non-system user -> ignored;
4) adb shell bmgr --user 0 activate true -> security exception;
adb shell bmgr --user 10 activate true -> security exception (work
profile);
adb shell bmgr --user 11 activate true/false -> creates/deletes activate
file and starts/stops the service
Change-Id: Ic77db9b8b2e5170dcf89bef863dac4713730797a