Make SkPicture/SkImageGenerator default to SkCodec

Make SkPicture/SkImageGenerator default to SkCodec

Remove reference to SkImageDecoder from SkPicture. Make the default
InstallPixelRefProc passed to CreateFromStream use
SkImageGenerator::NewFromEncoded instead.

Make SkImageGenerator::NewFromEncoded create an SkCodecImageGenerator.
Remove the old version that used SkImageDecoder.

Remove all versions of lazy_decode_bitmap/LazyDecodeBitmap. The default
now behaves lazily.

Update all clients to use the default.

Move SkImageDecoderGenerator into KtxTest.cpp, and use it directly.

This is a rebased version of:
https://codereview.chromium.org/1671193002/

TBR=reed@google.com

BUG=skia:4691
BUG=skia:4290
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1699183004

Review URL: https://codereview.chromium.org/1699183004