[APC] Introduce the flag to submit with the model by default

Introduces kSubmitWithEnterDuringPasswordChange feature flag. If
enabled, the APC flow will work like it used to before (try to submit
with Enter and fall back to submit with the model). Otherwise it will
always request the Submit button click using the model and won't try to
submit with the Enter key.
The flag is disabled by default, meaning that we're making the no Enter
key flow the default flow.

The CL also contains a small bug fix, changing timer.Reset to timer.Stop
to ensure that OnSubmissionDetectedOrTimeout is called only once.

Bug: 439496997
Change-Id: I37a159f04aa7283e481522933930a889d35beb31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6883691
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Commit-Queue: Anna Tsvirchkova <atsvirchkova@google.com>
Cr-Commit-Position: refs/heads/main@{#1509049}