Update imagedecoder documentation

Bug: 147839368
Test: No change in behavior

Update documentation in response to feedback and after reviewing
https://android.devsite.corp.google.com/ndk/reference/group/image-decoder.html#group___image_decoder_1ga8c3a19f8ae30a936a17b3879f849ea60

- Use @defgroup instead of @addtogroup, since this is the only file for
this group.
- Add a top-level description, including supported formats
- Describe the ANDROID_IMAGE_DECODER result enum, and use proper format
for the individual values so they will be added to the documentation
page.
- for createFromAAsset, specify that the AAsset can be closed *after*
deleting the AImageDecoder, as we do in createFromFd
- reformat some return lines to look more consistent
- Remove reference to animated images
- Move comment describing AImageDecoderHeaderInfo to before the typedef,
so it will be included in the documentation, and expand the comment
- More thorough description for getWidth/getHeight
- Move comment describing limitation on stride to stride (it had been on
the description for size)
- Clarify various comments, fix links, etc.

Change-Id: I26482ea8373081ee8d47455f7028f2bb6ec6a58e