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.cpp b/services/surfaceflinger/FlagManager.cpp
index 6d0dbc7..13b8a6c 100644
--- a/services/surfaceflinger/FlagManager.cpp
+++ b/services/surfaceflinger/FlagManager.cpp
@@ -110,6 +110,7 @@
     /// Trunk stable server flags ///
     DUMP_SERVER_FLAG(late_boot_misc2);
     DUMP_SERVER_FLAG(dont_skip_on_early);
+    DUMP_SERVER_FLAG(refresh_rate_overlay_on_external_display);
 
     /// Trunk stable readonly flags ///
     DUMP_READ_ONLY_FLAG(connected_display);
@@ -190,6 +191,7 @@
 
 /// Trunk stable server flags ///
 FLAG_MANAGER_SERVER_FLAG(late_boot_misc2, "")
+FLAG_MANAGER_SERVER_FLAG(refresh_rate_overlay_on_external_display, "")
 
 /// Exceptions ///
 bool FlagManager::dont_skip_on_early() const {