Let UserController mock OTAs

The system property "persist.pm.mock-upgrade" simulates an OTA
in certain contexts in the PackageManager. It would be convenient
if UserController also obeys it (to test things like ManagedProvisioning
responding correctly to OTAs).

Bug: 143200798
Test: Manual test of adb root; adb shell setprop persist.pm.mock-upgrade 1
Change-Id: I9326f70eb1864b356a1ff33e36bfeae7f6ad0ce6