Fix UserBackupManagerServiceTest failure on HSUM devices.

The UserBackupManagerService was hardcoded to operate on user 0, which caused testSetFrameworkSchedulingEnabled_enablesAndSchedulesBackups to fail on headless system user mode (HSUM) devices where the current user is not 0.

This change updates the service to be initialized with the correct user ID.
The corresponding test is also updated to dynamically fetch the current user at runtime, making the test robust for multi-user environments.

Flag: TEST_ONLY
Bug: 439491661
Test: atest UserBackupManagerServiceTest

Change-Id: I01c61f6fcc7b15e82bbebc18297b8f62ca7eee94