Use new suggestion type for prediction improvements

Adds a new suggestion to the filling suggestion's children that allows
filling the full form from the second-level menu. Also updates the
currently shown suggestion to show a placeholder error message if
retrieving prediction improvements wasn't successful.

Fixed: 359440030
Change-Id: I41b0a6f0efc58d3ff8213ee63af3065733d9041b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5836848
Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Wolfgang Billenstein <bwolfgang@google.com>
Cr-Commit-Position: refs/heads/main@{#1353366}