Fix backspace not working properly with hardware keyboards

[Susko3]

• Closes Backspace not working on external keyboard android osu#17803

Depends on:

• Fix pressing keys on a hardware keyboard showing the software keyboard #5274 (for textInputActive)

Android basically doesn't send the key up event for backspace. Really strange behaviour, I guess they expect text boxes not to care about key up events and instead rely on native key repeat.

Key up events works just find when text input is disabled (a.k.a. when OnCreateInputConnection() returns null).

[bdach]

I'll approve because I tested and it works, but I won't merge without a second approval because the fix is pretty hideous alll things considered.

All the above notwithstanding, the android input handling paradigm is likely terminally incompatible with physical keyboards, so I'm not exactly expecting there to be any saner way to do this than what's here.