EmulatorPkg/EmulatorPkg.dsc: override PCD default to avoid boot failure

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1215

This issue is caused by commit 5267926134d17e86672b84fd57b438f05ffa68e1.

The reason is this commit changed the condition of building page table
in DxeIpl. The code before it will only build page table for the sake
of PcdSetNxForStack. This commit added PcdImageProtectionPolicy and
PcdDxeNxMemoryProtectionPolicy into the logic. But the default value of
PcdImageProtectionPolicy is 02, which means the DxeIpl should build page
table. Due to the fact that EmulatorPkg doesn't support page table at all,
this will cause exception on Windows OS.

This patch solves this issue by setting PcdImageProtectionPolicy to 0
explicitly in EmulatorPkg.dsc.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>