Fix Backup/Restore confirmation dialog for HSUM multi-user scenarios.

This CL ensures correct backup/restore confirmation flow in
multi-user environments with non-system users, including HSUM.

Problem: Backup operations initiated for a secondary user would fail
because key logic was hardcoded to act on UserHandle.SYSTEM (user 0).
This led to two specific issues:
- The backup confirmation UI was launched for user 0 and subsequently blocked by ActivityTaskManager if a different user was in the foreground.
- Binder calls like hasBackupPassword() were incorrectly routed to user 0's profile, causing a SecurityException when called from an app running as a secondary user.

Solution:
- UserBackupManagerService now launches the confirmation UI for the user it is managing (mUserId).
- BackupManagerService directs password-related binder calls to the profile of the calling user.

Bug: 410032204
Bug: 420527847
Test: atest CtsBackupHostTestCases
Test: atest GtsBackupHostTestCases
Test: atest BackupManagerServiceTest
Flag: TEST_ONLY
Change-Id: I62b3f6de8061107e7233e9b76c0a077010b81c50