Fix ConcurrentModificationException in radio test

In broadcast radio service unit tests testing cancel, tune was also
called and invoked callback calling
RadioServiceUserController.getCurrentUser in a separate thread. This
causes concurrent modification exception when the next unit test
modifies the way mocking getCurrentUser if the current test does not
wait for that callback is completed. To test cancel in unit test,
calling tune in advance is not needed since it is duty of HAL to cancel
previous tuning operations. Thus, tune operations in unit test for
cancel can be removed.

Bug: 302236983
Test: atest com.android.server.broadcastradio
Change-Id: Ib42f302386aab2c5fa4d6159c14402b4adcc2de1