Replace EncodeBitmap with an interface.

Replace EncodeBitmap with an interface.

Gives more flexibility to the caller to decide whether to use the
encoded data returned by refEncodedData().

Provides an implementation that supports the old version of
SkPicture::serialize().

TODO: Update Chrome, so we can remove SK_LEGACY_ENCODE_BITMAP entirely

BUG=skia:3190

Review URL: https://codereview.chromium.org/784643002