MdeModulePkg: stop abusing EFI_HANDLE for keystroke notify registration

MdeModulePkg: stop abusing EFI_HANDLE for keystroke notify registration

EFI_REGISTER_KEYSTROKE_NOTIFY and EFI_UNREGISTER_KEYSTROKE_NOTIFY require
the notification handle to have type (VOID*). The notification handle has
nothing to do with the EFI_HANDLE type.

This change is a semantic fix; functionally, it's a no-op.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>