Store remaining aggregatable budget

This CL includes the following changes:

1. Store remaining budget instead of consumed budget.
2. Rename sources.num_aggregatable_reports to
sources.num_aggregatable_attribution_reports.
3. Change the type of the budget from int64_t to int, as the budget is
guaranteed to be in the int range.

The internals UI is updated to show remaining budget instead of
budget, and this is the only functionality change from this CL.

Change-Id: I76a1e64a8e44f3d531b234c9c2d7b78366bbd5a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5559825
Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1305360}