Refine get default value process for browser.

Refine get default value process for browser.
Before get default value for each questions, call ExtractConfig function to get the altcfg string for all formset. Later when question try to get default value from AltCfg string, just get the value from the saved altcfg string instead of call ExtractConfig function to get it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-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@16162 6f19259b-4bc3-4df7-8a09-765794883524