SkImageEncoder: simplify API

(re-land 248ff02 & 2cb6cb7, with changes)

  - Hide SkImageEncoder class in private header.
  - SkImageEncoder::Type becomes SkEncodedImageFormat
  - SkEncodedFormat becomes SkEncodedImageFormat
  - SkImageEncoder static functions replaced with
    single function EncodeImage()
  - utility wrappers for EncodeImage() are in
    sk_tool_utils.h

TODO: remove link-time registration mechanism.
TODO: clean up clients use of API and flip the flag.
TODO: implement EncodeImage() in chromeium/skia/ext

Change-Id: I47d451e50be4d5c6c130869c7fa7c2857243d9f0
Reviewed-on: https://skia-review.googlesource.com/4909
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-on: https://skia-review.googlesource.com/5186
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
diff --git a/src/codec/SkJpegCodec.h b/src/codec/SkJpegCodec.h
index 9e08c86..9a34aa6 100644
--- a/src/codec/SkJpegCodec.h
+++ b/src/codec/SkJpegCodec.h
@@ -51,8 +51,8 @@
 
     Result onGetYUV8Planes(const SkYUVSizeInfo& sizeInfo, void* planes[3]) override;
 
-    SkEncodedFormat onGetEncodedFormat() const override {
-        return kJPEG_SkEncodedFormat;
+    SkEncodedImageFormat onGetEncodedFormat() const override {
+        return SkEncodedImageFormat::kJPEG;
     }
 
     bool onRewind() override;