Don't autofill passwords in sandboxed contexts

If using the sandbox CSP or iframe tag, the web content is supposed to
be not trusted by the main resource origin. Therefore, we'd better
disable the password manager entirely so the untrusted web content
cannot exfiltrate passwords.

https://github.com/google/security-research/security/advisories/GHSA-mhhf-w9xw-pp9x

Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/1275>