WPB-4987 [Android] Add a button to jump to the latest message

PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

What's new in this PR?

Issues

We don't have a way in the app to quickly navigate to the bottom of the messages list, aka. jump to last message.

Solutions

Implement it by using Scaffold FAB embedded into messages list component, taking at the same time advantage that MessagesList component, passed down is a composable, so I was able to have it for free for Disabled message composer as well.

Using the outer scaffold was not an option after many tries, because it also considers the attachment options menu, making it almost impossible to customize the position dynamically.

Testing

Test Coverage (Optional)

• I have added automated test to this contribution

How to Test

Scroll so the last message is hidden, then the button should appear.

Attachments (Optional)

###Conversations with several messages and without scrollable content

###Read only Conversations

PR Post Submission Checklist for internal contributors (Optional)

• Wire's Github Workflow has automatically linked the PR to a JIRA issue

PR Post Merge Checklist for internal contributors

• If any soft of configuration variable was introduced by this PR, it has been added to the relevant documents and the CI jobs have been updated.

References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.