Rename SkBitmapRegionDecoder and Create function

We no longer need to worry about namespace
conflicts SkBitmapRegionDecoder in Android (which
we are replacing).

Additionally, the static Create() function does not
need to repeat the name BitmapRegionDecoder.

BUG=skia:

Review URL: https://codereview.chromium.org/1415243007
diff --git a/BUILD.public b/BUILD.public
index 9c66d3a..5d95d28 100644
--- a/BUILD.public
+++ b/BUILD.public
@@ -293,8 +293,8 @@
         "tools/SkBitmapRegionCanvas.h",
         "tools/SkBitmapRegionCodec.cpp",
         "tools/SkBitmapRegionCodec.h",
-        "tools/SkBitmapRegionDecoderInterface.cpp",
-        "tools/SkBitmapRegionDecoderInterface.h",
+        "tools/SkBitmapRegionDecoder.cpp",
+        "tools/SkBitmapRegionDecoder.h",
         "tools/SkBitmapRegionSampler.cpp",
         "tools/SkBitmapRegionSampler.h",
         "tools/flags/*.cpp",