Revert^2 "Set container to system for aconfig_declarations in AconfigFlags.bp"

This reverts commit 02287dc9e3dcf1fef015ca0db73d595800386f25.

Reason for revert: Performance regression is gone based on go/abtd verification, reintroduce the change.

Change-Id: I0dceae0d564612925f23002ac76fb60bbbe599f1
diff --git a/media/jni/playback_flags.aconfig b/media/jni/playback_flags.aconfig
index 2bb0ec5..9d927ec 100644
--- a/media/jni/playback_flags.aconfig
+++ b/media/jni/playback_flags.aconfig
@@ -1,4 +1,5 @@
 package: "com.android.media.playback.flags"
+container: "system"
 
 flag {
   name: "mediametadataretriever_default_rgba8888"