MdeModulePkg:Use safe string functions in UiApp.

MdeModulePkg:Use safe string functions in UiApp.

Replace the unsafe string  functions with the safe one in UiApp.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18214 6f19259b-4bc3-4df7-8a09-765794883524