Use Owners in PolicyVersionUpgrader

Currently PolicyVersionUpgrader operates on device_policies.xml
files, that contain ActiveAdmin objects and some of the device state
Owners data is accessed during the upgrade but cannot be modified
thus making it impossible to migrate a field from Owners to
ActiveAdmin. With this change PolicyVersionUpgrader will access
Owners data directly by reading and writing XML files, which allows
such field migrations and more.

Also extracted path getters from DPMS Injector to reduce duplication
(separate path getters were used in Owners) and make it possible to
override those paths once and for all in tests.

Bug: 218639412
Test: atest com.android.server.devicepolicy
Change-Id: I26013c00f3422522fbf0b6b119fe23953756c6d0