When the sidebar is connected to multiple guest frames it will send all
incoming annotations to all frames. The result is typically that the
annotation will anchor in one frame and orphan in the others. Depending
on what order this happens in, the annotation will non-deterministically
show up as an Annotation or Orphan in the sidebar.

In order to determine which frames an annotation should be sent to in
all cases, we'd either need the backend to return information about which
search URIs an annotation matches or make a separate search request for
each frame and record the associated frame with the results. This
will require some significant refactoring of the annotation search
service.

As an interim step, make `FrameSyncService` send annotations only to a
single frame based on matching URL, with a fallback to sending to the
main frame if there is no exact match. This will work as expected for
most pages, and is at least deterministic when it does fail. When we
have a solution for being able to match annotations to frames more
generally, we can adapt this code to use it.

This is a partial solution to https://github.com/hypothesis/client/issues/3992.