[enterprise-esim] Call new DPM API to check if subscription is managed

Call DevicePolicyManager.isSubscriptionEnterpriseManaged() instead of
directly checking the subscription's group owner.

Test: atest FrameworkTelephonyTests
Flag: android.app.admin.flags.enterprise_esim_using_carrier_privileges
Bug: 373466339

Change-Id: Id60a4bc084a85096b85d4a3d7f28de2e6ad99b06