Bug: 76461699
Bug: 76448408
Test: No change in behavior, no new tests
Add class level docs for ImageDecoder, including sample code. Update
wording to be more clear and less wordy. Fix broken attempts at using
sample code.
Incorporate advice at go/android-api-guidelines and
https://developers.google.com/style/api-reference-comments
Change-Id: Iaf1334993f6cd2d3f6e53d3fb70ef9c7a95c9a76