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/FlagManager.cpp b/services/surfaceflinger/FlagManager.cpp
index a8f05bb..5bbdfe9 100644
--- a/services/surfaceflinger/FlagManager.cpp
+++ b/services/surfaceflinger/FlagManager.cpp
@@ -102,6 +102,7 @@
     DUMP_FLAG(misc1);
     DUMP_FLAG(late_boot_misc2);
     DUMP_FLAG(vrr_config);
+    DUMP_FLAG(hotplug2);
 
 #undef DUMP_FLAG
 }
@@ -165,6 +166,7 @@
 FLAG_MANAGER_READ_ONLY_FLAG(enable_small_area_detection, "")
 FLAG_MANAGER_READ_ONLY_FLAG(misc1, "")
 FLAG_MANAGER_READ_ONLY_FLAG(vrr_config, "debug.sf.enable_vrr_config")
+FLAG_MANAGER_READ_ONLY_FLAG(hotplug2, "")
 
 /// Trunk stable server flags ///
 FLAG_MANAGER_SERVER_FLAG(late_boot_misc2, "")