IntelFrameworkModulePkg/LegacyUi: Get legacy options when open legacy form

IntelFrameworkModulePkg/LegacyUi: Get legacy options when open legacy form

The LegacyBootMaintUiLib depends on the LegacyBootManagerLib to realize its
functionality, the LegacyBootManagerLib may initialize after
LegacyBootMaintUiLib, so the functionality of LegacyBootMaintUiLib may
be incorrect. Now we fix this issue by executing the related codes when
opening the legacy forminstead in its the constructor function. Because
when opening the legacy form, the LegacyBootManagerLib must have been
initialized.

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>