Move SkBitmapRegionDecoder to include/android and src/android

This will expose the BitmapRegionDecoder API as a public include
and move the implementation to src.

This makes this code more naturally exposed in Android and easier
to test in DM and nanobench.

BUG=skia:

Review URL: https://codereview.chromium.org/1438873002
diff --git a/gyp/skia_lib.gyp b/gyp/skia_lib.gyp
index 8cb5fa1..b82809d 100644
--- a/gyp/skia_lib.gyp
+++ b/gyp/skia_lib.gyp
@@ -9,6 +9,7 @@
     'component_libs': [
       'core.gyp:core',
       'codec.gyp:codec',
+      'codec_android.gyp:codec_android',
       'effects.gyp:effects',
       'images.gyp:images',
       'opts.gyp:opts',