MdeModulePkg/BootMaintUiLib: Update menus when open BMM form

MdeModulePkg/BootMaintUiLib: Update menus when open BMM form

BootMaintenanceManagerUiLib depend on the LeagcyBootMaintUiLib to show the
legacy menus. So we need to do the actions related to LegacyUi in BMM
after the LeagcyBootMaintUiLib have been initialized. So now : 1). update
menus (including legacy menus), 2) re-scan boot options (including legacy
boot option) when opening the BMM form. We think when opening BMM form,
the LeagcyBootMaintUiLib 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>