Make ENSURE_VERIFY_APPS global even when set by PO.

Currently only device owner can set global user restrictions.
With this CL ENSURE_VERIFY_APPS will be global no matter who
enforces it, DO or PO.

To make it possible for system apps to check who enforces a
particular restriction in this case a new API method is added
to UserManager: getUserRestrictionSources which returns a list
of users who enforce the restriction.

Bug:31000521
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.UserRestrictionsTest (ag/1732744)
Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/devicepolicy/DevicePolicyManagerTest.java
Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/pm/UserRestrictionsUtilsTest.java
Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/devicepolicy/DevicePolicyManagerServiceMigrationTest.java
Test: installed M on a Nexus5x device, created a managed profile with some user restrictions, and checked that after upgrading M->O all restrictions are preserved and split correctly into base, global and local.
Change-Id: I543d3ec9ef0cf2b730da6f7406021c0bba43b785