[graphite] Add centralized app flag handling

This CL prepares the ground for adding additional Graphite-specific flags.

Bug: b/362718561
Change-Id: Id0ecdd599416e23ddcfa47e8fb13f42c52db291f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/894837
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed