[graphite] Clean up SkKeyHelpers a bit

The goals here are to:

reduce the use of magic numbers
make casts a bit safer

Bug: skia:12701
Change-Id: I6b1628fff4041bd0998060a757412b6a7260be1e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/496378
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed