Revert^2 "[legacy sf flag] - Remove legacyFrontEndEnabled"

This reverts commit ce6bde8dfa93038dd1063bae10b1f83eb1bc7e97.

Reason for revert: b/337894453 is now fixed and the original changes from ag/27000992 should no longer cause a regression in arc.PlayStore.betty_vm

Change-Id: I888a16b3137ba8f81d19b89c8ff291f4adf7d601
diff --git a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
index 85b1717..d40b2f8 100644
--- a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
+++ b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
@@ -691,6 +691,10 @@
         return mFlinger->initTransactionTraceWriter();
     }
 
+    // Needed since mLayerLifecycleManagerEnabled is false by default and must
+    // be enabled for tests to go through the new front end path.
+    void enableLayerLifecycleManager() { mFlinger->mLayerLifecycleManagerEnabled = true; }
+
     void notifyExpectedPresentIfRequired(PhysicalDisplayId displayId, Period vsyncPeriod,
                                          TimePoint expectedPresentTime, Fps frameInterval,
                                          std::optional<Period> timeoutOpt) {