usb/android_gadget: get serial string from emmc card cid

usb/android_gadget: get serial string from emmc card cid

BZ: 16897

This patch gets serial string from emmc card cid instead of
default serial string.

Signed-off-by: Liu, DongxingX <dongxingx.liu@intel.com>
Change-Id: I7c7e74fd74372e4dc799627f2989b2962b225d67
Reviewed-on: http://android.intel.com:8080/27194
Reviewed-by: Liu, DongxingX <dongxingx.liu@intel.com>
Reviewed-by: Gross, Mark <mark.gross@intel.com>
Reviewed-by: Wu, Hao <hao.wu@intel.com>
Tested-by: Dong, Chuanxiao <chuanxiao.dong@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>