[Maintenance] Made the SkData object within URLDataManager const

Make the SkData object (fData) within the UrlDataManager and UrlData classes const. This prevents the editing of the underlying data in the cache.

Change-Id: Id8e7b32fa13f039eecf527e5f6f2a9f7c8b8375c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1287516
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>