SF: add back accidently removed field

Add back is_fixed_read_only to
use_known_refresh_rate_for_fps_consistency feature.

Bug: 317189939
Test: presubmit
Change-Id: I8183e80cfc3711d0bf357f4f6db48deac61a4cb0
diff --git a/services/surfaceflinger/surfaceflinger_flags.aconfig b/services/surfaceflinger/surfaceflinger_flags.aconfig
index b65a2b3..79379fe 100644
--- a/services/surfaceflinger/surfaceflinger_flags.aconfig
+++ b/services/surfaceflinger/surfaceflinger_flags.aconfig
@@ -98,6 +98,7 @@
   namespace: "core_graphics"
   description: "Whether to use the closest known refresh rate to determine the fps consistency."
   bug: "299201319"
+  is_fixed_read_only: true
 }
 
 # This flag is broken.