Fix isUsbMassStorageConnected() will always return true

After UMS mounted, isUsbMassStorageConnected() will always return true even if USB is disconnected.
It's because mUmsEnabling will always be ture.

Change-Id: Ib24b2359ea2684eb0a9faeb880f383e87630e6e1

services/java/com/android/server/MountService.java

@@ -1155,7 +1155,7 @@ class MountService extends IMountService.Stub
 
     private void setUmsEnabling(boolean enable) {
         synchronized (mListeners) {
-            mUmsEnabling = true;
+            mUmsEnabling = enable;
         }
     }