Fix the issue the setting crashes after unmount SD card in landscape

When rotate the phone, mLastClickedMountToggle and mClickedMountPoint
would be destroied which made dialog access NULL pointer.

Change-Id: Ic96fce93828ca7a6f8749fad6a258eb47831e062
Author: Chuanxiao Dong <chuanxiao.dong@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: 26569