MdeModulePkg/UefiHiiLib: stop using EFI_HANDLE in place of EFI_HII_HANDLE
    
    HiiGetHiiHandles() returns an array of EFI_HII_HANDLEs, not EFI_HANDLEs.
    HiiGetString() takes an EFI_HII_HANDLE, not an EFI_HANDLE.
    
    This change is a no-op in practice; it's a semantic improvement.
    
    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>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>