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/bench/nanobench.cpp b/bench/nanobench.cpp
index 9b4db96..3587242 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -300,7 +300,7 @@
         SkDebugf("Can't write %s.\n", filename.c_str());
         return false;
     }
-    if (!SkImageEncoder::EncodeStream(&stream, bmp, SkImageEncoder::kPNG_Type, 100)) {
+    if (!SkEncodeImage(&stream, bmp, SkEncodedImageFormat::kPNG, 100)) {
         SkDebugf("Can't encode a PNG.\n");
         return false;
     }