Reject 4S key sharing requests after timeout

From riot-web created by [jryans](https://github.com/jryans): vector-im/riot-web#12664

After N seconds have passed since a device was verified, we should silently reject 4S key sharing requests from it.

(The value of N is still up for discussion.)

The issue names no files, tests, or entry points. Start by locating the Swift code that handles 4S key-sharing requests and device verification, then determine how the timeout should be represented. Done means requests are silently rejected after the agreed number of seconds, with coverage for the timeout behavior.

Written by the indexing model from the issue text.