Add container to .aconfig

Change-Id: I1770018c85b316a12faedd3dbe4e793f9520ca89
Test: m
Bug: 328444881
diff --git a/packages/EasterEgg/Android.bp b/packages/EasterEgg/Android.bp
index 6f4f9ca..ec1fe39 100644
--- a/packages/EasterEgg/Android.bp
+++ b/packages/EasterEgg/Android.bp
@@ -83,6 +83,7 @@
 aconfig_declarations {
     name: "easter_egg_flags",
     package: "com.android.egg.flags",
+    container: "system",
     srcs: [
         "easter_egg_flags.aconfig",
     ],