Prevent SecurityException during unit tests

Mock out displayOsd because it broadcasts an intent, which causes a
runtime error due to missing permissions on ADT-4.

Test: atest com.android.server.hdmi.SystemAudioAutoInitiationActionTest#testReceiveSystemAudioMode_systemAudioOn

Bug: 289908764
Change-Id: Ie3020da76c6d9996119493dc042de8d91c414623

services/tests/servicestests/src/com/android/server/hdmi/SystemAudioAutoInitiationActionTest.java

@@ -81,6 +81,20 @@ public class SystemAudioAutoInitiationActionTest {
             protected void writeStringSystemProperty(String key, String value) {
                 // do nothing
             }
+
+	    /**
+             * Override displayOsd to prevent it from broadcasting an intent, which
+             * can trigger a SecurityException.
+             */
+            @Override
+            void displayOsd(int messageId) {
+                // do nothing
+            }
+
+            @Override
+            void displayOsd(int messageId, int extra) {
+                // do nothing
+            }
         };
 
         mHdmiControlService.setIoLooper(myLooper);