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