Allow DeviceOwner to be set on "mainless" user on HSUM devices.

Currently on HSUM devices, the DeviceOwner can only be provisioned in
single-user mode in the main user, as that mode "emulates" a non-HSUM
device.

But the concept of main user is being deprecated, so instead it
should be set in the "first human user" of the device. Unfortunately,
there's no API to define which user is that, but since provisioning
is triggered by the Setup Wizard, it can be inferred from the
following characteristics:
  - User is the current user
  - User is admin
  - There are only 2 users in the device (the other one is system's)

In other words, the end result is the same - single-user mode DO can
only be set in the "first human user" - what changes is how that
user is identified.

Test: atest FrameworksMockingServicesTests --test-filter '.*UserManagerServiceMockedTest#testGetUserRemovabilityLocked.*'
Test: btest -it -s s GtsDevicePolicyTestCases:android.devicepolicy.gts.ManagedProvisioningRequiredAppsTest
Test: manual verification
Bug: 435271558
Bug: 435247804
Flag: android.app.admin.flags.device_owner_for_all

Change-Id: I72e23911f62291a9a72f45f729ff1873493a9242