Use SessionId of cached form.

To allow the Autofill framework to use cached results from a previous
prefill request, the form used to start an Autofill session must
have the same session id as the form from the prefill request. Only
then will the fields have the same virtual ids.

This CL adds logic to achieve that. It re-uses the session id of the
cached form iff:
- There is a cached form.
- The cached form is similar to the form the user has just focused.
- No bottom sheet has been shown before.

Otherwise it generated a new session id.

Bug: 1502097
Change-Id: I13a611acaf8547fc99137bfe7ff838b0373ce7e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5038706
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1226709}