Add missing container field to the aconfig flag file.

Bug: 328823994
Test: CI
Change-Id: I02dcf4e4879b1f22232ba72882867d3803454b12
diff --git a/media/audio/aconfig/audio_framework.aconfig b/media/audio/aconfig/audio_framework.aconfig
index 294e67d..f602f6c 100644
--- a/media/audio/aconfig/audio_framework.aconfig
+++ b/media/audio/aconfig/audio_framework.aconfig
@@ -4,6 +4,7 @@
 # Please add flags in alphabetical order.
 
 package: "android.media.audio"
+container: "system"
 
 flag {
     name: "auto_public_volume_api_hardening"