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.h b/services/surfaceflinger/FlagManager.h
index 1a68993..10784a7 100644
--- a/services/surfaceflinger/FlagManager.h
+++ b/services/surfaceflinger/FlagManager.h
@@ -53,6 +53,7 @@
     bool vrr_config() const;
     bool hotplug2() const;
     bool hdcp_level_hal() const;
+    bool multithreaded_present() const;
 
     /// Trunk stable server flags ///
     bool late_boot_misc2() const;