Fix AccessibilityManagerServiceTest

AccessibilityServiceConnection would send a message, and notify all
IAccessibilityServiceClient in the main thread. The message may not
be sent immediately. Using waitForIdleSync before verify to ensure
the target method is invoked.

Bug: 150345707
Test: atest FrameworksServicesTests:com.android.server.accessibility
Test: atest --rerun-until-failure 100 \
      AccessibilityManagerServiceTest#testOnSystemActionsChanged
Change-Id: I6a796772e00f55d6523e1f12703c17b0fd931f9c

services/tests/servicestests/src/com/android/server/accessibility/AccessibilityManagerServiceTest.java

@@ -189,6 +189,7 @@ public class AccessibilityManagerServiceTest extends AndroidTestCase {
     public void testOnSystemActionsChanged() throws Exception {
         setupAccessibilityServiceConnection();
         mA11yms.notifySystemActionsChangedLocked(mUserState);
+        InstrumentationRegistry.getInstrumentation().waitForIdleSync();
         verify(mMockServiceClient).onSystemActionsChanged();
     }
 }