Fixed UserManagerTest so it works on Automotive.

Prior to this CL, this test case would fail miserably on automotive because it would try to remove
the current user (and it would also fail on tests for managed profiles).

It was tested using the automotive emulator (lunch gcar_hl_emu_x86-userdebug).

Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest
Bug: 146435179

Change-Id: Ie498d39b2f441618f4e5cad68ea9ce18856842d4