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

Note that D3DCommon can't use DX11HRWrap or DX12HRWrap since it's shared between them.

While trying to work on adding audiodump support for CLI, I was alerted that it was important to first try moving the DSP configs to the new config before continuing, as that makes it substantially easier to write clean code to add such a feature.

This commit aims to allow for Dolphin to only rely on the new config for DSP-related settings.