Refactor Layer/ColorLayer into siblings

New class hierarchy should look like:

           Layer (abstract)
           /   \
          /     \
        Buffer  Color
        Layer   Layer

Note: LayerBuffer is what previously was referred to by class Layer

Test: Transaction_test.cpp/manual test

Change-Id: I372a27d533e08371da37be811e4372404f7787bd
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index 3f8ab3f..7115ad6 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -14,6 +14,7 @@
     FrameTracker.cpp \
     GpuService.cpp \
     Layer.cpp \
+    BufferLayer.cpp \
     ColorLayer.cpp \
     LayerRejecter.cpp \
     LayerVector.cpp \