Call DPM from RestrictedPreferenceHelper

Call DevicePolicyManager.getEnforcingAdminsForPolicy API to check for
user restrictions on RestrictedPreferenceHelper.

Update mAttrUserRestriction field from String to an inner class
Restriction. This way, we'll have a clear way of representing user
restriction or a device policy which can be enforced by admins.

Test: atest com.android.settingslib.RestrictedPreferenceHelperTest
Flag: android.app.admin.flags.policy_transparency_refactor_enabled
Bug: 414733570

Change-Id: I0dfd6490ffee18f6f9ad3420e107607f9d3633be