Auto lock private space on device restarts

This change enables auto-locking private space after device boots by
calling setQuietModeEnabled whenever UserManagerService initializes. We
add the change behind a separate feature flag dedicated to control the
release of this change.

Test: Manually by restarting device when private space is added
Bug: 296993385
Change-Id: If78b10dce5f69ad9412af964fafa805664fc408b