[graphite] Begin refactoring paint key creation

Bug: 251493570
Change-Id: Id92358016150e1cc4f73da2368e37c13c04a8902
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/754020
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed