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
diff --git a/services/java/com/android/server/MountService.java b/services/java/com/android/server/MountService.java
index 8cf8f6a..85bb3aa 100644
--- a/services/java/com/android/server/MountService.java
+++ b/services/java/com/android/server/MountService.java
@@ -1155,7 +1155,7 @@
 
     private void setUmsEnabling(boolean enable) {
         synchronized (mListeners) {
-            mUmsEnabling = true;
+            mUmsEnabling = enable;
         }
     }