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/BufferQueueLayer.cpp b/services/surfaceflinger/BufferQueueLayer.cpp
index b236fd2..2fa8936 100644
--- a/services/surfaceflinger/BufferQueueLayer.cpp
+++ b/services/surfaceflinger/BufferQueueLayer.cpp
@@ -126,6 +126,11 @@
 
 bool BufferQueueLayer::fenceHasSignaled() const {
     Mutex::Autolock lock(mQueueItemLock);
+
+    if (SurfaceFlinger::enableLatchUnsignaled) {
+        return true;
+    }
+
     if (!hasFrameUpdate()) {
         return true;
     }