encrypting to untrusted devices means no degradition of security
compared to not encrypting at all. Trust status display (shield) is made
independently at a later stage.

until now problems with verifying the call (omemo or DTLS missing) would
just be another app failure. This commit displays verifications problems as
their own thing.

fixes #4065

There might be corner cases where it is required to use self signed
certificates. However there should be no corner cases where it is
required to use a wrong domain name. This commit swaps out the
MemorizingHostnameVerifier that let users accept wrong domains with the
standard XmppDomainVerifier.

closes #4066

Until now Conversations and Quicksy would only disply the dialog that explains
why we want contact read permissions after the user rejected the request once

(following Android design guidelines and `shouldShowRequestPermissionRationale()`)

However for PlayStore policy this is no longer enough and the app needs to
explain and ask for consent before starting to upload the data.

This commit now displays the explain dialog immediately before asking for the
first time.

some VPN apps are broken on Android 7.1 (and below?)

fixes #4058

fixes #4057

fixes #4053

fixes #4049

people with half a brain will implicitly follow those guidelines any way and
provide the information requested in the issue template.

The vast majority of people have ignored the issue template in the past anyway.

Thank you to @ailicic for figuring out the new API.

Closes #4050

Upon accepting a video call on a device that can not establish a video track on
its own (for example by not having a camera), displaying the video enable/disable
button would fail. This commit defaults this button to disabled.