Use system_ext as the container of the aconfig flags in Settings

Bug: 332723689
Test: Run `adb shell printflags` and verify the flags defined in aconfig
folders show up
Flags: EXEMPT aconfig file only changes

Change-Id: If0b497117c303a0bd0b366819c90bd11b41232c3

aconfig/development/settings_core_flag_declarations.aconfig

@@ -1,5 +1,5 @@
 package: "com.android.settings.development"
-container: "system"
+container: "system_ext"
 
 flag {
   name: "a2dp_offload_codec_extensibility_settings"