Set the default value of `Config::MAIN_WASAPI_DEVICE` to `default`
instead of `Default`. This fixes an issue where `AudioPane`'s `Output
Device` combo would be blank if `WASAPI` was selected and the user had
never changed the value of `Output Device`.

We don't have to worry about backward compatibility with users who have
`Default` in their config because config values aren't written unless
they've been changed at some point from the default, and the combo has
always saved `default` instead of `Default`.

Audio still worked during emulation in this situation because the
fallback for an unrecognized device name is the default device.

To help prevent similar situations in the future references to
`MAIN_WASAPI_DEVICE`'s default value now call `GetDefaultValue` instead
of hardcoding the expected default, or use the new helper function
`Config::IsDefaultValue`.

Check the return value of calls to `GetBuffer` and stop the `WASAPI
handler` sound thread if they fail.

This prevents a crash due to a null pointer dereference if `GetBuffer`
is unable to retrieve a buffer, which could be triggered during
emulation by disabling the selected WASAPI output device in the Windows
Sound settings.

AudioCommon: Remove unused includes