UsbSetting: The behavior is not right when enable/disable MTP/PTP with Accessory board connected.

Add accessory flag to track accessory status. Disable MTP/PTP button
when Usb is working in accessory mode.

Change-Id: I3edcd2a7eb281db3bb9d345429643b77a237812e
Author: Wu, Hao <hao.wu@intel.com>
Signed-off-by: Xiaokang Qin <xiaokang.qin@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 27200