Launch voice assistant by the KEYCODE_MEDIA_PLAY_PAUSE

Android's headset spec is update to deprecate the KEYCODE_HEADSETHOOK,
and the KEYCODE_MEDIA_PLAY_PAUSE will be used instead. So we need to
launch voice assistant by both the KEYCODE_MEDIA_PLAY_PAUSE and
the KEYCODE_HEADSETHOOK.

See following docucument for the details
https://source.android.com/devices/accessories/headset/usb-headset-spec#software-mapping

Bug: 36575213
Test: Manual test (injects long press of the KEYCODE_MEDIA_PLAY_PAUSE
    and confirmed that it launches voice activity)
Change-Id: I3dc2500ca5bb3d9d0ba3104246a19b84e731b46f
diff --git a/services/core/java/com/android/server/media/MediaSessionService.java b/services/core/java/com/android/server/media/MediaSessionService.java
index 0db1b5d..2cb9cdd4 100644
--- a/services/core/java/com/android/server/media/MediaSessionService.java
+++ b/services/core/java/com/android/server/media/MediaSessionService.java
@@ -1505,7 +1505,8 @@
         }
 
         private boolean isVoiceKey(int keyCode) {
-            return keyCode == KeyEvent.KEYCODE_HEADSETHOOK;
+            return keyCode == KeyEvent.KEYCODE_HEADSETHOOK
+                    || keyCode == KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE;
         }
 
         private boolean isUserSetupComplete() {