[graphite] Switch PaintParamsKey over to a builder/result model

This will leave the PaintParamsKeys more clearly immutable and manage more of the cruft of key creation (e.g., beginBlock/endBlock pairing).

Bug: skia:12701
Change-Id: I1944397465f845e974e950f47cedb90c3f1895b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/506881
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
38 files changed