Add "is_fixed_read_only" in aconfig for cache_if_source_crop_layer_only_moved flag.

Bug: 305718400
Change-Id: I076fa6b5ed26d7909cd80ac2c1df8038f72b65cb
Test: builds
diff --git a/services/surfaceflinger/surfaceflinger_flags.aconfig b/services/surfaceflinger/surfaceflinger_flags.aconfig
index fcb52c7..ca53c3a 100644
--- a/services/surfaceflinger/surfaceflinger_flags.aconfig
+++ b/services/surfaceflinger/surfaceflinger_flags.aconfig
@@ -98,6 +98,7 @@
   namespace: "core_graphics"
   description: "do not flatten layers if source crop is only moved"
   bug: "305718400"
+  is_fixed_read_only: true
 }
 
 flag {