DPMS Upgrade: Lazy user listing

Minor clean-ups to the DPMS upgrade mechanism:
* Rename isUserDeviceOwner to isDeviceOwner.
* Only collect the users list if there is an upgrade to be performed.

Bug: 181009337
Test: atest FrameworksServicesTests:PolicyVersionUpgraderTest
Change-Id: I0cb79f1e569e630c4f2863e80f0e4b81b5b41fcc