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