Make flag foreground_audio_control read only

Some platforms access AppOpsManager before SettingsProvider is
up, so flag needs to be read-only

Bug: 327865409
Test: build/boot see bug

Change-Id: Ief47cb8b19b1788af946daa3cb1ab03ab5c0de32
diff --git a/media/audio/aconfig/audio_framework.aconfig b/media/audio/aconfig/audio_framework.aconfig
index 525dceb..4bda863 100644
--- a/media/audio/aconfig/audio_framework.aconfig
+++ b/media/audio/aconfig/audio_framework.aconfig
@@ -91,4 +91,5 @@
     namespace: "media_audio"
     description: "Limit access to volume and ringer SDK APIs in AudioManager"
     bug: "296232417"
+    is_fixed_read_only: true
 }