hwc2: Do not flush on empty layer stack.

-- Some applications can take time to queue buffers during switches ie;
   HDR to Non-HDR, Secure to Non-secure etc; Even though layer stack
   is empty it is OK to display previous frame.

-- Composer Client explicitly clears layers upon SurfaceFlinger death.
   Since client isn't active, chances of getting a new frame is very unlikey.
   Trigger flush so that content gets cleared.

Change-Id: If91135b3d5ea6d0f8842c7ef75547a68002c70ff
CRs-Fixed: 2489780
diff --git a/composer/hwc_display_builtin.cpp b/composer/hwc_display_builtin.cpp
index b4c2290..63bd2d8 100644
--- a/composer/hwc_display_builtin.cpp
+++ b/composer/hwc_display_builtin.cpp
@@ -203,7 +203,7 @@
 
   if (layer_set_.empty()) {
     // Avoid flush for Command mode panel.
-    flush_ = !IsDisplayCommandMode();
+    flush_ = !client_connected_;
     validated_ = true;
     return status;
   }