[Scanner] Don't stop scanner fully on max forms

[dbajpeyi]

Reviewer: @shakyShane @GioSensation
Asana: https://app.asana.com/0/0/1207840563867787/f

Description

Originally, #574 fixed an issue that destructed forms if it didn't have enough eligible inputs that could be filled out. This way such forms won't take up the form pool (of max 30 forms), and avoid getting into a situation where an actually useful form won't get into the pool. However, the PR also introduced a problem that if there were many lonely inputs (e.g in ~60 this test form), the scanner created forms that self destructed over and over again.

This PR tries a different approach:

1. Reverts the original PR mentioned above (except few aesthetic changes),
2. Renames scanShadow -> scanOnClick and extends it to not only work for elements within shadow trees, but also general inputs that might need to be scanned again.
3. Modifies the stopScanner so that it can take an additional param that allows keeping form listeners,
4. Performance tests seems to not show any regression.

Steps to test

Originally reported sites: https://scanmalta.com/shop

1. Build the browser locally with autofill, go to https://scanmalta.com/shop,
2. click on the login button and if you have credentials saved you will see that autofill is prompted.
3. Submit button statistics are back to how it was before scanner: discard Form instances with no classified inputs #574. This is unfortunate, but makes sense because this solution is less general.

   Submit button statistics:
       638 detected (47% false positive)
       389 manually identified (14% false negative)