Enable backup activation for all users in HSUM via flag

This change introduces a mechanism to enable backup and restore
functionality for all users in Headless System User Mode (HSUM)
using a new aconfig flag, without requiring individual activation files.

Flag: android.multiuser.backup_activated_for_all_users
Bug: 374830726
Bug: 406114361
Test: BackupManagerServiceTest
Change-Id: I2576dfee0d48035a2578f9c840dae63df08b98ca