Fix UserManagerServiceTest mocking test

One existing test did not take into account the possibility of being
able to switch to the headless system user. Depending on the value of
config_canSwitchToHeadlessSystemUser on test device, this test may or
may not pass.

This CL explicitly sets the config value, to get consistent test result.

Bug: 391913130
Flag: EXEMPT bugfix
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Change-Id: I7cbe4c1786fc770faadecd213efb4aceaf50f016