Add container to .aconfig

Change-Id: I404c5ad928d22d82ffae613d7efe84993dfdc9af
Test: m
Bug: 328444881
diff --git a/libs/WindowManager/Shell/aconfig/Android.bp b/libs/WindowManager/Shell/aconfig/Android.bp
index 1a98ffc..7f8f57b 100644
--- a/libs/WindowManager/Shell/aconfig/Android.bp
+++ b/libs/WindowManager/Shell/aconfig/Android.bp
@@ -1,6 +1,7 @@
 aconfig_declarations {
     name: "com_android_wm_shell_flags",
     package: "com.android.wm.shell",
+    container: "system",
     srcs: [
         "multitasking.aconfig",
     ],