Snap: use platform cert

This is needed to be able to put Snap into it's own selinux domain.

Change-Id: I23f73723a9b62ee67b6d0f234898828eba945962
diff --git a/Android.mk b/Android.mk
index 7ad043b..b9ee73b 100755
--- a/Android.mk
+++ b/Android.mk
@@ -38,7 +38,8 @@
 
 LOCAL_PACKAGE_NAME := Snap
 LOCAL_PRIVILEGED_MODULE := true
-LOCAL_PRIVATE_PLATFORM_APIS:=true
+LOCAL_PRIVATE_PLATFORM_APIS := true
+LOCAL_CERTIFICATE := platform
 
 LOCAL_AAPT_FLAGS += --rename-manifest-package org.lineageos.snap