iOS home indicator obstructs components behind it

[frenzibyte]

We apply an override to bottom edge safe area in OsuGameIOS, making the game extend to the home indicator. This causes the home indicator to obstruct components behind it, such as song select footer buttons or chat overlay text box etc.

The initial goal was to add extensive handling of safe area so the home indicator gets its own space without just a black box surrounding it, but further discussions halted this approach pending proper direction from the design team first so the final result does not look awkward.

One temporary direction here is to remove the override and make this user toggleable similar to the horizontal edges. Preview without the override, aka "letterbox":

[frenzibyte]

cc @peppy

[frenzibyte]

Also cc @ppy/team-design for commentary on the screenshots above and how to better fill the black gap around the home indicator in the second batch of screenshots so it doesn't look silly like it is right now. There are other areas to discuss as well but let's start with the above first.