Set the container field of aconfig flags

Please see go/aconfig-flag-containers for more context

Test: m
Bug: 312769710
Change-Id: I95efda5d266ad45202501d82d256d9e9f8e7826a
diff --git a/aconfig/Android.bp b/aconfig/Android.bp
index dc30a35..5413601 100644
--- a/aconfig/Android.bp
+++ b/aconfig/Android.bp
@@ -20,6 +20,7 @@
 aconfig_declarations {
     name: "com_android_launcher3_flags",
     package: "com.android.launcher3",
+    container: "system",
     srcs: ["**/*.aconfig"],
 }