Added menu item for summarization feature

This CL adds a menu item for the experimental summarization feature.
This feature was already available as a toolbar shortcut, its icon and
strings are placeholders.

Additionally it modifies AiAssistantService to fall back to sending an
intent whenever a feature isn't available in the system provider.

Bug: 397527693
Change-Id: I5b5cf8e51557dda559b48b5b32b17d74e716a9f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6279010
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Salvador Guerrero Ramos <salg@google.com>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1424217}