Make testRemoveUserByHandle_ThrowsException fail faster.

Currently UserManagerTest#testRemoveUserByHandle_ThrowsException uses
the helper 'removeUser' method which registers for a
BlockingBroadcastReceiver and since we're not expecting a broadcast
it only fails after the timeout. This CL removes that unnecessary step
and decreases this test method's duration from 3 minutes to 1 ms.

Test: atest UserManagerTest#testRemoveUserByHandle_ThrowsException
Change-Id: I9302054cb53b1e0027bb2bcb9da74d98f390f2ed