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/public.bzl b/public.bzl
index 55f40f8..970d756 100644
--- a/public.bzl
+++ b/public.bzl
@@ -484,6 +484,7 @@
     "src/effects",
     "src/effects/gradients",
     "src/fonts",
+    "src/images",
     "src/pathops",
     "src/pipe/utils",
     "src/ports",
@@ -600,6 +601,7 @@
     "SK_SUPPORT_LEGACY_ACCESSBITMAP",
     "SK_SUPPORT_LEGACY_CLIP_REGIONOPS",
     "SK_SUPPORT_LEGACY_XFERMODE_IS_PUBLIC",
+    "SK_SUPPORT_LEGACY_IMAGE_ENCODER_CLASS",
     # Temporarily Disable analytic AA for Google3
     "SK_NO_ANALYTIC_AA",
 ]