Enable refreshrate indicator on external display.

- Add flag for it as well.

Bug: 301647974
Test: play with external monitor; adb shell device_config put
core_graphics
com.android.graphics.surfaceflinger.flags.refresh_rate_overlay_on_external_display
true

Change-Id: I530a7a74b2ec7be1c1a35a1310c87e073764c77a
diff --git a/services/surfaceflinger/FlagManager.h b/services/surfaceflinger/FlagManager.h
index cefce9b..e3e4f80 100644
--- a/services/surfaceflinger/FlagManager.h
+++ b/services/surfaceflinger/FlagManager.h
@@ -49,6 +49,7 @@
     /// Trunk stable server flags ///
     bool late_boot_misc2() const;
     bool dont_skip_on_early() const;
+    bool refresh_rate_overlay_on_external_display() const;
 
     /// Trunk stable readonly flags ///
     bool connected_display() const;