Replace privacy-math global configuration with parameter

It is easier to reason about the system without the test-only mutable
state of these globals.

Strictly it isn't even necessary for privacy math to know about source
type anymore, but we will make that simplification in a separate CL.

Unfortunately we cannot currently make the same change for trigger-state
cardinality because that value is needed for parsing attribution scopes.

This CL contains no intentional user-visible behavioral changes.

Change-Id: I567fab56e328377cf4a35d03da5e20ba49c3c9d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5861180
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1355426}