In contact list and conversation screen's header, display real profile
image loaded for each contact.

Add TestAvatarCreator implementation that adds random avatar images for
testing contacts.

Because the MessageTracker in briar initializes the GroupCount timestamp
to the current time when the contact is being added, we cannot have
realistic test data in the database where the GroupCount timestamp
matches that of the last message in the chat history. This is a dirty
workaround to make this possible.

Nico authored 3 years ago and Sebastian committed 3 years ago

So far without messages, therefore solves #113 only partly.

Sub-task of #12.

properly instantiate, initialize and clear view models at the right point in time using a straightforward API mimicking Android's ViewModelProvider

Also, add another test that automatically connects both started app
instances.