[VCN] Add function to start manual fill immediately

Add function to the mediator handler to trigger manual fill. This
will allow mediator to notify the coordinator when mediator tries to
retrieve suggestion but gets a "show manual fill instead" decision from
the components layer.

Bug: 343517197
Change-Id: I3981a1330ca0e420e545c70d78e975b74e3cba02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5586399


Reviewed-by: Vincent Boisselle <vincb@google.com>
Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Siyu An <siyua@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313665}