This specific issue was already addressed by https://github.com/dolphin-emu/dolphin/pull/11635
though I felt like there was something more we could do, and wasn't too happy with the
likelihood of devices update calls being skipped (due to `m_devices_population_mutex` being locked).

Fixes bug: https://bugs.dolphin-emu.org/issues/12744

Before https://github.com/dolphin-emu/dolphin/commit/e1e3db13baabefa89991388d37db0bb260c4f535
the ControllerInterface m_devices_mutex was "wrongfully" locked for the whole Initialize() call, which included the first device population refresh,
this has the unwanted (accidental) consequence of often preventing the different pads (GC Pad, Wii Contollers, ...) input configs from loading
until that mutex was released (the input config defaults loading was blocked in EmulatedController::LoadDefaults()), which meant that the devices
population would often have the time to finish adding its first device, which would then be selected as default device (by design, the first device
added to the CI is the default default device, usually the "Keyboard and Mouse" device).

After the commit mentioned above removed the unnecessary m_devices_mutex calls, the default default device would fail to load (be found)
causing the default input mappings, which are specifically written for the default default device on every platform, to not be bound to any
physical device input, breaking input on new dolphin installations (until a user tried to customize the default device manually).

Default devices are now always added synchronously to avoid the problem, and so they should in the future (I added comments and warnings to help with that)

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.

This makes the device ID assigning code common to all backends, by
moving it to AddDevice() instead of copy-pasting or replicating
the logic in the backends.

Also, to prepare for hotplugging, instead of relying on a name usage
count, the new ID assigning system always starts from ID 0 and tries
to assign the first ID that is not used.

Previously, the devices vector would be passed to all backends. They
would then manually push_back to it to add new devices. This was fine
but caused issues when trying to add synchronisation.

Instead, backends now call AddDevice() to fill m_devices so that it is
not accessible from the outside.