MdeModulePkg/Variable: Check EFI_MEMORY_RUNTIME attribute before setting it

MdeModulePkg/Variable: Check EFI_MEMORY_RUNTIME attribute before setting it

Set the EFI_MEMORY_RUNTIME attribute in FtwNotificationEvent() only if
the attribute is not already present. This will ensure that the attributes
set by the platform drivers (e.g Ovmf pflash) is not lost.

Cc: Dong Eric <eric.dong@intel.com>
Cc: Justen Jordan L <jordan.l.justen@intel.com>
Cc: Zeng Star <star.zeng@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Reviewed-by: Star Zeng <star.zeng@intel.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>