Invoke autofill on compose dialog focus lost

1) Introduces an autofill client entrypoint to show a fading
popup (saved state notification).
2) Creates scaffolding required to detect focus lost then invoke the
new autofill entrypoint with the field/form ids.

Bug: b/303500278
Change-Id: I3dff203c3e5bb4ef7250759fb8509318c1cc442f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5309574
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: edchin <edchin@google.com>
Cr-Commit-Position: refs/heads/main@{#1263518}