Clean up physical coupling

Narrow the use of #include directives in hwui, replacing with forward
declarations where straightforward. Speeds compiles; doesn't do any
restructuring of code.

Change-Id: Icac2baffb5896f55d8c6718e9bd9d4bfa02d3ca0
diff --git a/libs/hwui/SkiaShader.cpp b/libs/hwui/SkiaShader.cpp
index c672bc4..dff19a9 100644
--- a/libs/hwui/SkiaShader.cpp
+++ b/libs/hwui/SkiaShader.cpp
@@ -21,6 +21,7 @@
 #include <SkMatrix.h>
 
 #include "Caches.h"
+#include "Extensions.h"
 #include "Layer.h"
 #include "Matrix.h"
 #include "SkiaShader.h"