Provide callback instead of doing UI tasks of calling app

[rfc2822]

See bitfireAT/davx5-ose#1294 (comment):

• cert4android shouldn't show the TrustCertificateActivity (and probably the notification) by itself, but instead should execute a callback that's provided by the calling application
• cert4android then doesn't need to track "in foreground" anymore.

[rfc2822]

Closed for later