Fullscreen on Android doesn't hide system bars

In Tor Browser 13.0.7, when you go on fullscreen on Android, system bars persist.

So, as a matter of fact, it isn't a real fullscreen.

STR:

1. Open a video (tried on Invidious instances, but also other sites)
2. Go to fullscreen
3. Notice that the both the bar with clock/notifications and the "navigation" bar are still visible (I'm using gesture-based navigation, so I don't have a bar with buttons, but a bar with a horizontal line)

If you open the same site on Firefox (but without a custom config such as ours), fullscreen works as expected.

It might have started after the security backport.

Tested on my Pixel 4a, I've seen the problem both on 13.0.7 and on 13.5a3.

/cc @ma1