Add a control to toggle fullscreen mode

My current problem with fullscreen mode are:
* it's hard to turn it off temporarily, as my tablet don't always detect when I'm trying to drag from the bottom of the screen because of the screen protector
* I usually want to do multiple things, such as "flag and suspend" or "show answer and select answer", but the border layout disappear after the first thing

I think the best solution would be to add a control that would toggle full screen.
Ensuring it's easy to get the buttons (and that they remain until toggle again), and that it would be easy to stop having the button if we need more room on the screen.

I tried to take a look at it in https://github.com/Arthur-Milchior/Anki-Android/tree/fullscreen, and I must admit it seems not trivial, given that the settings are read during the creation of the reviewer in order to decide which layout to use, which does not seems like something that can be easily changed, at least not without recreating the view.

However, we can force the button to reappear, in fullscreen mode. Which means that at least half of what I want is possible. Ensuring the button reappear. But I have not yet understood how this work