Test flaky test: Always wait for handle task to complete

- Previous logic of checking queue to wait or not has race as queue can be
  emptied while posted task is not complete.
- Do not check queue and always wait for new post to complete.

Bug: 129896850
Test: Run the test many times
Change-Id: Ie0e4447cf997cbe343c7e00a8dfd0757b061b547

services/tests/servicestests/src/com/android/server/am/UserControllerTest.java

@@ -484,9 +484,6 @@ public class UserControllerTest {
 
     private void waitForHandlerToComplete(Handler handler, long waitTimeMs)
             throws InterruptedException {
-        if (!handler.hasMessagesOrCallbacks()) { // if nothing queued, do not wait.
-            return;
-        }
         final Object lock = new Object();
         synchronized (lock) {
             handler.post(() -> {