EmulatorPkg: Add VariableFlashInfoLib

EmulatorPkg: Add VariableFlashInfoLib

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3479

Adds an instance of VariableFlashInfoLib to the platform build as
it is a new library class introduced in MdeModulePkg.

Cc: Andrew Fish <afish@apple.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Nickle Wang <nickle.wang@hpe.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Abner Chang <abner.chang@hpe.com>