REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.
Abstract VariableWriteServiceInitializeDxe/Smm from
FtwNotificationEvent/SmmFtwNotificationEvent, then
VariableWriteServiceInitializeDxe/Smm could be not aware
the NV storage is real or emulated.
This patch prepares for adding emulated variable NV mode
support in VariableRuntimeDxe.