Add a way to iterate over a specific op type

Also split the OpBuffer tests out from CanvasOpTests

Test: this
Change-Id: I0fbc726d108bfa4333c01daf7aedacca8716e6ae
diff --git a/libs/hwui/Android.bp b/libs/hwui/Android.bp
index 0533aa6..9d82f63 100644
--- a/libs/hwui/Android.bp
+++ b/libs/hwui/Android.bp
@@ -618,6 +618,7 @@
         "tests/unit/LayerUpdateQueueTests.cpp",
         "tests/unit/LinearAllocatorTests.cpp",
         "tests/unit/MatrixTests.cpp",
+        "tests/unit/OpBufferTests.cpp",
         "tests/unit/PathInterpolatorTests.cpp",
         "tests/unit/RenderNodeDrawableTests.cpp",
         "tests/unit/RenderNodeTests.cpp",