Update SystemUI R8 documentation

Update guidance based on R8 optimizations now being opt-out.

Test: n/a
Bug: 203472868
Change-Id: I721dfb0ac5be2956c5a29d9b5ef63da36272a28c
diff --git a/packages/SystemUI/Android.bp b/packages/SystemUI/Android.bp
index cd3a722..2845323 100644
--- a/packages/SystemUI/Android.bp
+++ b/packages/SystemUI/Android.bp
@@ -239,11 +239,9 @@
     plugins: ["dagger2-compiler"],
 }
 
-// Opt-in config for optimizing the SystemUI target using R8.
-// Enabled via `export SYSTEMUI_OPTIMIZE_JAVA=true`, or explicitly in Make via
-// the `SOONG_CONFIG_ANDROID_SYSTEMUI_OPTIMIZE_JAVA` variable.
-// TODO(b/203472868): Enable optimizations by default after stabilizing and
-// building out retrace infrastructure.
+// Opt-out config for optimizing the SystemUI target using R8.
+// Disabled via `export SYSTEMUI_OPTIMIZE_JAVA=false`, or explicitly in Make via
+// `SYSTEMUI_OPTIMIZE_JAVA := false`.
 soong_config_module_type {
     name: "systemui_optimized_java_defaults",
     module_type: "java_defaults",