Break Layer class into Gl and Vulkan subclasses

Test: manual testing
Change-Id: Ibd2beed39de3ac6da7448e96496253cfe427dfbb
diff --git a/libs/hwui/Android.mk b/libs/hwui/Android.mk
index 18e59e5..f24c1fb 100644
--- a/libs/hwui/Android.mk
+++ b/libs/hwui/Android.mk
@@ -75,6 +75,7 @@
     FrameInfo.cpp \
     FrameInfoVisualizer.cpp \
     GammaFontRenderer.cpp \
+    GlLayer.cpp \
     GlopBuilder.cpp \
     GpuMemoryTracker.cpp \
     GradientCache.cpp \