Revert "[legacy sf flag] - Remove legacyFrontEndEnabled"

This reverts commit 11e197a2e438f0ea16066ff6118294a5fa3b26a3.

Reason for revert: caused regression in arc.PlayStore.betty_vm see http://b/337352234

Change-Id: I9ba883f0190b82ac1e00ed47ace8bec4e8a46fa6
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index e04bf17..44fa806 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -1486,6 +1486,7 @@
     bool mPowerHintSessionEnabled;
 
     bool mLayerLifecycleManagerEnabled = false;
+    bool mLegacyFrontEndEnabled = true;
 
     frontend::LayerLifecycleManager mLayerLifecycleManager GUARDED_BY(kMainThreadContext);
     frontend::LayerHierarchyBuilder mLayerHierarchyBuilder GUARDED_BY(kMainThreadContext);