UsbMidiDevice: fix MIDI flush()

The proxy MidiReceiver in the USB device was not forwarding the flush
command to the event scheduler.

Bug: 25511696
Change-Id: I6a4759b71bc8f9ae3e20aed1238f62a2ed405e24
Signed-off-by: Phil Burk <philburk@google.com>

services/usb/java/com/android/server/usb/UsbMidiDevice.java

@@ -127,6 +127,14 @@ public final class UsbMidiDevice implements Closeable {
         public void setReceiver(MidiReceiver receiver) {
             mReceiver = receiver;
         }
+
+        @Override
+        public void onFlush() throws IOException {
+            MidiReceiver receiver = mReceiver;
+            if (receiver != null) {
+                receiver.flush();
+            }
+        }
     }
 
     public static UsbMidiDevice create(Context context, Bundle properties, int card, int device) {