Activity gain focus failed in InputMethodStressTest

After the test is completed, the notification is not closed,
so that subsequent test cannot get the focus.

Close notification in tearDown to solve the issue.

Bug: 241723484
Test: run InputMethodStressTest pass

Change-Id: I876173e83488e18a884d8bc180f9681e726fff1d

tests/InputMethodStressTest/src/com/android/inputmethod/stresstest/NotificationTest.java

@@ -102,6 +102,7 @@ public final class NotificationTest {
     @After
     public void tearDown() {
         mNotificationManager.cancelAll();
+        mUiDevice.pressHome();
     }
 
     @Test