MidiService: synchronize mDeviceConnections

Try to prevent a NullPointerException caused
by scanning the array while devices were being removed
by another thread.

Bug: 193278370
Test: atest CtsMidiTestCases
Change-Id: I8560cf6f39654037d5ccdc0f6922fa8bdb719c2f

services/midi/java/com/android/server/midi/MidiService.java

@@ -350,8 +350,10 @@ public class MidiService extends IMidiManager.Stub {
             }
 
             if (mDeviceConnections != null) {
-                for (DeviceConnection connection : mDeviceConnections) {
-                    connection.notifyClient(server);
+                synchronized (mDeviceConnections) {
+                    for (DeviceConnection connection : mDeviceConnections) {
+                        connection.notifyClient(server);
+                    }
                 }
             }
         }