Move test(Set|Revoke)UserAdmin* to UserManagerServiceMockedTest

The new test case for revokeUserAdmin() being added in CL:33795062 needs
to be implemented in UserManagerServiceMockedTest to guarantee that
there is only one full admin user on the test environment, while the
existing test cases for this method live in UserManagerTest.
Move all the tests for revokeUserAdmin() from UserManagerTest to
UserManagerServiceMockedTest so that the new test case can be added in
the same class as the existing tests.
Also move the tests for setUserAdmin() for consistency.

Bug: 411222385
Test: atest FrameworksMockingServicesTests:UserManagerServiceMockedTest
Flag: TEST_ONLY

Change-Id: Ic07d08c6fedb90316e7c4e1cfb87f47c636083eb