Add libgui bq_consumer_attach_callback flag

Add a flag for enabling IProducerListener callback when the consumer
attaches a buffer.

Bug: 353202582
Flag: EXEMPT adding flag
Test: n/a
Merged-In: I3268cb82fdc7d194f8b1864b8a72147ff51e3973
Merged-In: Ieaa25ed05d04a33de47de7ee3296e4bf1dde7a26
Change-Id: Ieaa25ed05d04a33de47de7ee3296e4bf1dde7a26

Author: Sungtak Lee

libs/gui/libgui_flags.aconfig

@@ -9,6 +9,14 @@ flag {
   is_fixed_read_only: true
 }
 
+flag {
+  name: "bq_consumer_attach_callback"
+  namespace: "core_graphics"
+  description: "Controls IProducerListener to have consumer side attach callback"
+  bug: "353202582"
+  is_fixed_read_only: true
+} # bq_consumer_attach_callback
+
 flag {
   name: "frametimestamps_previousrelease"
   namespace: "core_graphics"