MdeModulePkg: Avoid update keyword string package.

MdeModulePkg: Avoid update keyword string package.

When add new string, only update normal string package, avoid update keyword string package.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@hp.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17430 6f19259b-4bc3-4df7-8a09-765794883524