Simplify references to `VerificationRequest` (#11045)
* Use new `VerificationRequest.getQRCodeBytes()`
* Use new `VerificationRequest.otherDeviceId`
* Remove references to `VerificationRequest.channel`
* Replace references to `VerificationRequest.{requesting,receiving}UserId`
Normally these are guarded by `request.initiatedByMe` so we can trivially
replace it with `request.otherUserId` or `client.getUserId()`. In one place we
actually need to apply some logic.
* increase test coverage
* Even more test coverage
* Even more test coverage
