Make UM.hasBaseUserRestriction @TestApi

The devicepolicy CTS test now needs to know when a restrictions is due
to devicepolicy or whether it is a 'base' restriction enacted by
UserManager. Therefore UM.hasBaseUserRestriction is opened up for the
CTS test.

Bug: 147111703
Test: atest com.android.cts.devicepolicy.UserRestrictionsTest
Change-Id: I0dd5256d36249a8dfe23ae10acf344d9de5fb2ca