[cert4android] Move cert4android UI tasks to app

[rfc2822]

Right now, when a users decision on whether to trust a certificate is needed, cert4android will show TrustCertificateActivity and/or a notification if it's allowed to. This is error prone (see #1313) so we want to move the responsibility of showing a notification and/or the certificate trust decision dialog to DAVx5.

When implementing, also consider #861 (reply in thread)

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Depends on bitfireAT/cert4android#49

[github-actions]

This PR/issue depends on:

• Provide callback instead of doing UI tasks of calling app cert4android#49

[rfc2822]

We should also better handle the customCerts[Ui] build flag: if it's false, we should immediately reject questionable certificates.

Guess it would be better to have an option for that in cert4android?