MdeModulePkg:Set default string value

MdeModulePkg:Set default string value

Previously construct Default Value in AltResp for string opcode is String Id,
Now change it to string value if the string opcode has default value.
This change is compatible for SetupBrowser to handle.And add default value
for string in DriverSample to test this case.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18337 6f19259b-4bc3-4df7-8a09-765794883524