Re-introduce debug.sf.latch_unsignaled

This pretty much reverts this commit: Ia453eaf895554adbb2fa9c11c2bc8609e1037ea5.
Some devices may find this helpful to improve performance, but there is a risk of pipeline stall from misbehaving apps.

Bug: 173694115
Test: manual
Change-Id: I48c3095061dc106fa08d36b8ef17eccb04fb6eb1
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 1d6f016..6cff608 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -270,6 +270,8 @@
     // being treated as native display brightness
     static bool enableSdrDimming;
 
+    static bool enableLatchUnsignaled;
+
     // must be called before clients can connect
     void init() ANDROID_API;