Make UserManagerServiceTest use a single way to set flags

EnableFlags/DisableFlags is the recommended way to set flags in test.
The existing mSetFlagsRule.enableFlag is deprecated.

This CL converts existing code to the new convention. Plus, it makes the
import of the flags cleaner.

Bug: 380122753
Flag: EXEMPT test only
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Change-Id: Ic47fe54022b38bef29fb28f1a60f7dc37bd99d5a