EmulatorPkg/EmulatorPkg.dsc: Change PCDs type about status code

EmulatorPkg/EmulatorPkg.dsc: Change PCDs type about status code

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2786

Since the type of PcdStatusCodeUseSerial and PcdStatusCodeUseMemory
in MdeModulePkg.dec are changed, so change them from
PcdsFeatureFlag to PcdsFixedAtBuild in dsc file.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Signed-off-by: Ming Tan <ming.tan@intel.com>