BootMaintenanceManagerUiLib: change code for customization.

BootMaintenanceManagerUiLib: change code for customization.

Change file arrangement to let user easy customize the BMM UI.
After this change:
1. UiCustomizeBMMPage function in BootMaintenanceManagerCustomizedUi.c
use to let user customize BMM first page menus. UiBMMCallbackHandler
function use to handle the callback for the added menus.
2. BootMaintenanceManagerCustomizedUiSupport.c file used to add support
 functions used by BootMaintenanceManagerCustomizedUi.c file.
3. After this change, only above two files need to be updated if BMM
first page menus needed to be changed.

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