Add getEnforcingAdminsForPolicy method to DPM

Add the new method that retrieves EnforcingAdmins list for the given
policy identifier to DevicePolicyManager and to
DevicePolicyManagerService. The implementation in DPMS is work in
progress and will be completed in the follow-up CL for legacy policies.

Add helper method to EnforcingAdmin to check if admin is supervision
admin. Move the admin role constants to EnforcingAdmin class to make it
more consistent.

Test: atest FrameworksServicesTests:com.android.server.devicepolicy.DevicePolicyManagerTest
Bug: 414733570
Flag: EXEMPT code not used yet

Change-Id: I410ec99749c2734eaafe2556aac9e06e1cd33e9f