Up until now, there have been two settings on Android that stored the
selected Wii Remote extension: the normal one that's also used on PC,
and a SharedPreferences one that's used by the overlay controls to
determine what controls to show. It is possible for these two to end up
out of sync, and my input changes have made that more likely to happen.

To fix this, let's rework how the overlay controller setting works.
We don't want it to encode the currently selected Wii Remote extension.
However, we can't simply get rid of the setting, because for some Wii
games we need the ability to switch between a GameCube controller and a
Wii Remote. What this commit does is give the user the option to select
any of the 4 GameCube controllers and any of the 4 Wii Remotes. (Before,
controllers 2-4 weren't available in the overlay.) Could be useful for
things like the Psycho Mantis fight in Metal Gear Solid. I'm also
switching from SharedPreferences to Dolphin.ini while I'm at it.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Ensures that all class state is initialized to valid values.