Split skia_defaults into skia_flags_defaults.

skia_flags_defaults be used in ag/40819920 (WIP CL) to avoid inheriting
the srcs.

BUG=529738725

Change-Id: I8cc6e8bad27e4b403efdb5e7a823af5e4ed644bd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1287027
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>