Fix bmgr whitelist on HSUM devices

The bmgr whitelist command is broken on headless system user mode (HSUM) devices because it incorrectly performs user-specific checks for a device-scoped property.

This change fixes the following issues:
1. The bmgr tool checks if the backup service is active for the current user before fetching the whitelist.
   On HSUM devices, this check fails for the default user (user 0), causing the command to exit prematurely.
2. The BackupManagerService.getTransportWhitelist() method also contains a user-specific check, which fails for the headless system user and returns null.

Flag: EXEMPT bug fix
Bug: 442476405
Test: gts-tradefed run commandAndExit gts -m GtsOsTestCases -t com.google.android.os.gts.SysConfigTest#testBackupTransportWhitelist
Change-Id: I5f257d7f375ce89fa5dab44012f54aae940a74da