SF: Remove BufferStateLayer class

This CL removes the BufferStateLayer class and some of the references to
"BufferStateLayer" in factory and utility method names. In order to keep
the diff smaller, I'll send out a separate CL to remove the remaining
references.

Bug: 238781169
Test: refactor, existing tests pass

Change-Id: I62fe00caa87e109435da62329e817c19cfb96ead
diff --git a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
index 1ce6e18..983fac2 100644
--- a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
+++ b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
@@ -30,7 +30,6 @@
 #include <ftl/fake_guard.h>
 #include <gui/ScreenCaptureResults.h>
 
-#include "BufferStateLayer.h"
 #include "DisplayDevice.h"
 #include "FakeVsyncConfiguration.h"
 #include "FrameTracer/FrameTracer.h"
@@ -120,9 +119,7 @@
         return compositionengine::impl::createCompositionEngine();
     }
 
-    sp<BufferStateLayer> createBufferStateLayer(const LayerCreationArgs&) override {
-        return nullptr;
-    }
+    sp<Layer> createBufferStateLayer(const LayerCreationArgs&) override { return nullptr; }
 
     sp<Layer> createEffectLayer(const LayerCreationArgs&) override { return nullptr; }