Move our sepolicy to system_ext

Each app domain should be defined under the installed partition's
sepolicy. Since PixelSystemService is under system_ext, we should
not define our sepolicy under vendor.

Test: manually check if the error message is gone
Fix: 293806319
Change-Id: Ifec3e58c44ef1b2bc522f44b874b0635edc38c12
diff --git a/pixelsystemservice/pixelsystemservice.mk b/pixelsystemservice/pixelsystemservice.mk
index fcabe89..4a27110 100644
--- a/pixelsystemservice/pixelsystemservice.mk
+++ b/pixelsystemservice/pixelsystemservice.mk
@@ -2,4 +2,4 @@
 PRODUCT_PACKAGES += \
        PersistentBackgroundServices
 
-BOARD_VENDOR_SEPOLICY_DIRS += device/google/gs-common/pixelsystemservice/sepolicy
+SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS += device/google/gs-common/pixelsystemservice/sepolicy