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/libs/gui/Android.bp b/libs/gui/Android.bp
index eb4d3df..4c3cc6c 100644
--- a/libs/gui/Android.bp
+++ b/libs/gui/Android.bp
@@ -23,6 +23,7 @@
 aconfig_declarations {
     name: "libgui_flags",
     package: "com.android.graphics.libgui.flags",
+    container: "system",
     srcs: ["libgui_flags.aconfig"],
 }