Move AlphaFilterCanvas to new API.

The AlphaFilterCanvas is only used when it will make a copy of the paint
anyway. Remove the need to keep track of copy on write semantics when
they are never used.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: I20c0d618db7d1a76756bd4c16fe686ce7fb44f38
diff --git a/libs/hwui/pipeline/skia/RenderNodeDrawable.cpp b/libs/hwui/pipeline/skia/RenderNodeDrawable.cpp
index 1bd30eb..e65fe3a 100644
--- a/libs/hwui/pipeline/skia/RenderNodeDrawable.cpp
+++ b/libs/hwui/pipeline/skia/RenderNodeDrawable.cpp
@@ -187,17 +187,13 @@
     AlphaFilterCanvas(SkCanvas* canvas, float alpha) : SkPaintFilterCanvas(canvas), mAlpha(alpha) {}
 
 protected:
-    bool onFilter(SkTCopyOnFirstWrite<SkPaint>* paint, Type t) const override {
-        std::optional<SkPaint> defaultPaint;
-        if (!*paint) {
-            paint->init(defaultPaint.emplace());
-        }
-        paint->writable()->setAlpha((uint8_t)(*paint)->getAlpha() * mAlpha);
+    bool onFilter(SkPaint& paint) const override {
+        paint.setAlpha((uint8_t)paint.getAlpha() * mAlpha);
         return true;
     }
     void onDrawDrawable(SkDrawable* drawable, const SkMatrix* matrix) override {
         // We unroll the drawable using "this" canvas, so that draw calls contained inside will
-        // get their alpha applied. THe default SkPaintFilterCanvas::onDrawDrawable does not unroll.
+        // get their alpha applied. The default SkPaintFilterCanvas::onDrawDrawable does not unroll.
         drawable->draw(this, matrix);
     }