Refactor the "week" duration to be a global variable

[csharrison]

The spec makes extensive use of a hardcoded "week" aspect of the privacy privacy unit, while acknowledging that it is arbitrary. Given that, it seems pretty likely we might get feedback that some other limit is preferrable. This topic came up a bunch in the TPAC WICG breakout (minutes).

For now, I think it's probably wise to hardcode this once in a global variable and refer to the variable throughout so any changes can be made seamlessly.

Separately, we might consider if different browsers could set different durations, but that's outside the scope of this issue.