[iOS][MF] Prevent leaking suggestions

Makes explicit when the custom keyboard views are paused.
Nils last suggestions on reset.
Removes the accessory view from non web urls.
Organizes code in the the form activity handler.
Stops using |lastSuggestions| to gate view controller updates,
|[pause / continue]CustomViewController| is used instead.

Bug: 845472
Change-Id: I751015f9bd2b7b085c15a10fd41fd949dc1fa672
Reviewed-on: https://chromium-review.googlesource.com/c/1355179
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612280}
3 files changed