SF: Add a readonly flag for the hotplug2 hal api feature

Bug: 303460805
Test: adb shell dumpsys SurfaceFlinger | grep hotplug2 -- verify the
flag shows up.

Change-Id: I9abdbec32e8517d82234c2a52d4586d4e6ed12b8
diff --git a/services/surfaceflinger/surfaceflinger_flags.aconfig b/services/surfaceflinger/surfaceflinger_flags.aconfig
index 19d194f..fec450e 100644
--- a/services/surfaceflinger/surfaceflinger_flags.aconfig
+++ b/services/surfaceflinger/surfaceflinger_flags.aconfig
@@ -52,4 +52,12 @@
   description: "Feature flag for SmallAreaDetection"
   bug: "283055450"
   is_fixed_read_only: true
-}
\ No newline at end of file
+}
+
+flag {
+  name: "hotplug2"
+  namespace: "core_graphics"
+  description: "Feature flag for using hotplug2 HAL API"
+  bug: "303460805"
+  is_fixed_read_only: true
+}