BootMaintenanceMangerUiLib: Save mode info for later use.

BootMaintenanceMangerUiLib: Save mode info for later use.

In current code, we use different output modes for boot phase
and setup phase. When split BootMaintenanceMangerUiLib from
UiApp code, we not add logic to save the boot phase mode info
which will be used later. This change add this logic.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>