SecurityPkg: Use FileExplorerLib in SecureBootConfigDxe

SecurityPkg: Use FileExplorerLib in SecureBootConfigDxe

Using existing library FileExplorerLib to replace the same
logic in SecureBootConfigDxe to make the code clear. After using
FileExplorerLib, the UI behavior for enroll PK will change,
previously when select one PK file, commit/discard changes will
return to Device Manager,press ESC will return to FileExplorer.
Now using FileExplorerLib the behavior will keep same with
enroll KEK/DB/..., commit/discard changes will return to Custom
Secure Boot Options form and ESC will return to PK options form.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>