Enable DPCs to download subscriptions via EuiccManager APIs

Ensures that DPCs can download subscription on company-owned devices.
Also tags the subscription as being "admin-owned", to disambiguate
between dpc downloaded subscriptions and user downloaded subscriptions.
This is used to determine which subscriptions an admin can
control.

Bug: 293456669
Test: btest android.devicepolicy.cts.TelephonyTest
Test: Manual with testdpc
Change-Id: I15fcd8d2ff5c749b1abd3caa673716ab01db6935