Bug 1015006 - Detect USB cable unplugged and set sharing to false. r=qDot

If the USB cable is pulled after we call SetIsSharing(true) but before we actually initiate the
sharing operation, then the volume doesn't change state, and mIsSharing never gets set back to
false. So we detect this condition now and force mIsSharing to be false when the USB cable is
unplugged.