Add support in AffiliationPrefetcher to observe sources.

The AffiliationPrefetcher owns multiple AffiliationSources. This CL adds
support for the AffiliationPrefetcher to observe add/remove events from
all its sources and prefetch or cancel affiliation data accordingly.

Bug: b/328037758
Change-Id: Ide22332958361bd0ad31eba5b2086da0ecd1b336
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5370017


Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Norge Vizcay <vizcay@google.com>
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Cr-Commit-Position: refs/heads/main@{#1283141}