Use FABs instead of top bar actions for progressing through flows

### Your use case

#### What would you like to do?

Change the create room/invite flow so that the primary forward navigation is performed using a floating action button, instead of a "Next"/"Create"/etc textual action in the top bar. (I'm not sure if there are any other flows in the app that might need this treatment, too..)

#### Why would you like to do it?

It's more consistent with the material guidelines and how other Android apps work. FABs are intended to put important actions in easy reach (you don't have to stretch all the way to the top of the screen), so we should them for such actions :)

### Have you considered any alternatives?

We could, of course, leave it as is. It's workable, it's just a little _off_ for a native Android app.

### Additional context

_No response_

### Are you willing to provide a PR?

No