SF FlagManager: Add a method for checking multithreaded_present

Factored out of Ib9d074671e32c95875ef7e0791dd95d6e595e47a to avoid
more potential rebases. That CL will use the flag.

Bug: 241285491
Bug: 259132483
Test: adb shell dumpsys SurfaceFlinger | grep multithreaded_present
  -- verify flag shows up default false.

Change-Id: Id3ba46cbc0c9487bf3b29e7b9d4de58e5ee8e459
diff --git a/services/surfaceflinger/FlagManager.cpp b/services/surfaceflinger/FlagManager.cpp
index 11a5e0d..1f8a3f5 100644
--- a/services/surfaceflinger/FlagManager.cpp
+++ b/services/surfaceflinger/FlagManager.cpp
@@ -104,6 +104,7 @@
     DUMP_FLAG(vrr_config);
     DUMP_FLAG(hotplug2);
     DUMP_FLAG(hdcp_level_hal);
+    DUMP_FLAG(multithreaded_present);
 
 #undef DUMP_FLAG
 }
@@ -169,6 +170,7 @@
 FLAG_MANAGER_READ_ONLY_FLAG(vrr_config, "debug.sf.enable_vrr_config")
 FLAG_MANAGER_READ_ONLY_FLAG(hotplug2, "")
 FLAG_MANAGER_READ_ONLY_FLAG(hdcp_level_hal, "")
+FLAG_MANAGER_READ_ONLY_FLAG(multithreaded_present, "debug.sf.multithreaded_present")
 
 /// Trunk stable server flags ///
 FLAG_MANAGER_SERVER_FLAG(late_boot_misc2, "")