MdeModulePkg/HiiDatabaseDxe: Support EfiVarStore to get AltCfg from Driver

MdeModulePkg/HiiDatabaseDxe: Support EfiVarStore to get AltCfg from Driver

Allow EfiVarStore to get <AltResp> from Hii Driver, and enhance code logic
in MergeDefaultString function to get a full AltCfgResp.
The logic in function MergeDefaultString after enhancement:
(1) If there are no <AltResp> in AltCfgResp, merge the <AltResp> in
DefaultAltCfgResp to AltCfgResp.
(2) If there are <AltResp> in AltCfgResp, for the same <AltConfigHdr>, if
the <ConfigElement> already in AltCfgResp, don't need to merge from
DefaultAltCfgResp, else merge the <ConfigElement> in the DefaultAltCfgResp
to the related <AltResp> in AltCfgResp.
AltCfgResp: Generated by Driver.
DefaultAltCfgResp: Generated by HiiDatabase.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
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>