SkSpan cleanup
SkSpan cleanup

* Check index in operator[]
* Use begin/end in const versions
* Remove returning SkSpan{} when empty for first, last, and subspan.

Change-Id: I0348ff8effb01e9175c5afaa52ddb0724f86218d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303259
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Commit-Queue: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia