SkJpegMetadataDecoderImpl: Move to separate file

The metadata decoding for JPEG files will be getting larger due to
the inclusion of ISO gainmap support. Move to a separate file to
avoid creating more clutter.

Bug: b/338342146
Change-Id: Ic28c7ba1aa36edb488906dc227df8570818c3af3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/849356
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>
6 files changed