Protected Audience: store k-anon keys & update times in interest group table

Instead of storing the k-anonymity status & update time for each k-anon
key, store this information as new columns in the interest_groups table:
store one k-anon update time for each interest group and the
keys for that interest group that *are* k-anonymous. The k-anon table
will only store k-anon keys that have been joined and the time they were
last joined. Keys that are not k-anonymous and have never been joined
will not be stored anywhere.

K-anon queries are already done one interest group at a time, and if
there is not a large overlap in keys between interest groups, keys will
not be queried much more often than necessary. The database will no
longer enforce that the same k-anon key will have the same k-anonymity
status for every interest group. K-anon updates in the database will not
be performed per-key, but will be performed per-interest group.

After each interest group update or join, the database will return the
last k-anon update time, the keys that were newly introduced by the
update/join, and all keys associated with the interest group. The
InterestGroupKAnonymityManager will update the new keys only if the last
update time was recent, or it will update all keys if the last update
time was not recent. The update time is only updated when all keys are
updated.

Bug: 338035176
Change-Id: Id807d4ec44cc7c0ae29e0023ddfd5b4f55828343
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5521957
Reviewed-by: Russ Hamilton <behamilton@google.com>
Commit-Queue: Abigail Katcoff <abigailkatcoff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1308325}