MtpServer: Do not attempt to stop MTP if it has not been started

Fixes an ANR that occurred if we got a USB disconnected event
before the MTP service had started.

Bug: 4118033

Change-Id: I6cad4281a5911a9926cae923f34d3a6bf98346c5
Signed-off-by: Mike Lockwood <lockwood@android.com>

media/java/android/mtp/MtpServer.java

@@ -24,6 +24,9 @@ import android.util.Log;
  */
 public class MtpServer {
 
+    private final Object mLock = new Object();
+    private boolean mStarted;
+
     private static final String TAG = "MtpServer";
 
     static {
@@ -35,11 +38,19 @@ public class MtpServer {
     }
 
     public void start() {
-        native_start();
+        synchronized (mLock) {
+            native_start();
+            mStarted = true;
+        }
     }
 
     public void stop() {
-        native_stop();
+        synchronized (mLock) {
+            if (mStarted) {
+                native_stop();
+                mStarted = false;
+            }
+        }
     }
 
     public void sendObjectAdded(int handle) {