MdeModulePkg/BMMUI: Show "Change Boot/Driver order" page correctly

MdeModulePkg/BMMUI: Show "Change Boot/Driver order" page correctly

When user enter the "Change Boot Order" page, the BootOptionOrder in
BmmFakeNvData may maintain some uncommitted data which are not saved
in "BootOrder" Variable and BootOptionMenu. So we should not always get
the BootOptionOrder through the function GetBootOrder, it will
result in incorrect UI behaviors. When the BootOptionOrder has not been
saved, we should use the BootOptionOrder in current BmmFakeNvData.

Cc: Laszlo Ersek <lersek@redhat.com>
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>