Add extra for DPM for EnforcingAdmin

The extra will be used for policy transparency on RestrictedLockUtils.
It's marked as SystemApi because RestrictedLockUtils is a mainline
module.

Flag: android.app.admin.flags.enforcing_admin_extra_enabled
Bug: 414733570
Test: None
FCRS_CODE : mdyqt2vlizwx7v

Change-Id: I27f40ff512c1e9b1fc504092da87cc41a31a9564