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 true.

- https://review.source.android.com//#change,16403

Change-Id: Ib24b2359ea2684eb0a9faeb880f383e87630e6e1

author  Tony Wu <Tony_Wu@acer.com.tw>
commit  fc711251