Set the container field of aconfig flags

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

Test: m
Bug: 312769710
Change-Id: Ibe1f32691a2fb10a267b580766abcde621a596ff
diff --git a/services/surfaceflinger/Android.bp b/services/surfaceflinger/Android.bp
index 0989863..dcef9a3 100644
--- a/services/surfaceflinger/Android.bp
+++ b/services/surfaceflinger/Android.bp
@@ -10,6 +10,7 @@
 aconfig_declarations {
     name: "surfaceflinger_flags",
     package: "com.android.graphics.surfaceflinger.flags",
+    container: "system",
     srcs: ["surfaceflinger_flags.aconfig"],
 }