MdeModulePkg:Clear the screen before booting the boot option

MdeModulePkg:Clear the screen before booting the boot option

When do UiApp remodeling task,miss clearing the screen before booting
the selected boot option,so cause some behaviors change.Now add the code
to fix this issue.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19441 6f19259b-4bc3-4df7-8a09-765794883524