UserManagerTest: Only register a receiver when it's needed.

The tests involve removing users and the broadcasts generated
by these events could block the tests from receiving other
broadcasts, so only register a receiver when it is needed
and unregister once it is not needed anymore.

Bug: 260158381
Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest
Change-Id: I9169fe3d34400ece137f585462c18b2a883bc82b