switch from external/sepolicy to system/sepolicy

Switch from using external/sepolicy to system/sepolicy.
Bug: 27875478

default.xml

@@ -273,8 +273,7 @@
   <project path="external/safe-iop" name="platform/external/safe-iop" groups="pdk" />
   <project path="external/scrypt" name="platform/external/scrypt" groups="pdk" />
   <project path="external/selinux" name="platform/external/selinux" groups="pdk" />
-  <project path="external/shflags" name="platform/external/shflags" />
-  <project path="external/sepolicy" name="platform/external/sepolicy" groups="pdk" />
+  <project path="external/shflags" name="platform/external/shflags" groups="pdk" />
   <project path="external/sfntly" name="platform/external/sfntly" groups="pdk,qcom_msm8x26" />
   <project path="external/skia" name="platform/external/skia" groups="pdk,qcom_msm8x26" />
   <project path="external/sl4a" name="platform/external/sl4a" />
@@ -518,6 +517,7 @@
   <project path="system/netd" name="platform/system/netd" groups="pdk" />
   <project path="system/nvram" name="platform/system/nvram" />
   <project path="system/security" name="platform/system/security" groups="pdk" />
+  <project path="system/sepolicy" name="platform/system/sepolicy" groups="pdk" />
   <project path="system/tools/aidl" name="platform/system/tools/aidl" groups="pdk-cw-fs,pdk-fs" />
   <project path="system/tpm" name="platform/system/tpm" />
   <project path="system/update_engine" name="platform/system/update_engine" />