PolicyVersionUpgrader: fix ERROR + stacktrace on first boot

On first boot it is expected that the device_policies_version file
doesn't exist yet, so properly handle this case.

Bug: 280155081
Test: Booted Cuttlefish and verified that logcat no longer contains
      "DevicePolicyManager: Error reading version" and the stacktrace,
      but does contain "DevicePolicyManager: Upgrading from version 0"
Change-Id: Ib0322997848fd86734a9e1e660a60274cfc8f15e