Update javadocs for ImageDecoder and related

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