MdeModulePkg/BMUiLib: Check reset requirement before exiting UiApp

V2: Refine the comments.

In UI page, some configuration change may require system reset.
BootManagerUiLib misses this check before exiting UiApp to boot
other boot options. Now add the check.

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