Handle legacy policies in getEnforcingAdminsForPolicy

Handle the legacy policies that policy transparency UIs query for in
DPM::getEnforcingAdminsForPolicy method. Add identifiers for these
policies to DevicePolicyIdentifiers file. They will stay hidden for now.

Test: atest FrameworksServicesTests:com.android.server.devicepolicy.DevicePolicyManagerTest#getEnforcingAdminsForPolicy_legacyPolicy
Bug: 414733570
Flag: EXEMPT code not used yet
Change-Id: I15efa8552ff5a39cad1ed6f3b6a3c5250bd10c45