Validate SkDescriptor length alignment

SkDescriptor expects all lengths to be 4 byte aligned, but the
deserialization code path doesn't enforce this.

Update isValid() checks to reject unaligned lengths.

Bug: 490254124
Change-Id: Ie179750307df29a98aba67b008fa29fdce488bc4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1184276
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
(cherry picked from commit 22ef657fd3be7042d3253295c7c01d66b0fec404)
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1187499