It is possible that a remote DECLINE message arrives short before the
user responds to the introduction.
This will cause a ProtocolStateException which (for now) is just caught
and a generic (existing) error message will be shown.

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.

This also makes the defaults consistent with Android versions below O.

In practise, this is not noticeable in the UI.
Only when the database is congested, it should become visible and
prevent a crash when the sound setting is clicked.