This PR adds a ```NO_AUDIO``` constant switch to the DesktopGL,
WindowsDX, Android, and iOS .csproj of MonoGame.

The reasoning for this switch is to better accommodate third-party sound
engines like FMOD or Wwise by entirely disabling audio and any audio
dependency that could interfere with or corrupt FMOD/Wwise (while not
changing the API).

While MonoGame already has a mechanism that doesn't initialize audio
unless you use it, there are still possible annoyances:
- OpenAL/NVorbis/XAudio are enforced as a dependency, even if
```PrivateAssets``` is used. Therefore users using FMOD or Wwise are
still required to ship with those binaries even if they don't use the
MonoGame Audio namespace at all (otherwise the app won't start);
- the non-initialization mechanism doesn't guarantee that OpenAL or
XAudio aren't loaded (it entrusts static initializers for not doing so,
which can't be assumed to be reliable throughout all .NET runtimes)

The ```NO_AUDIO``` switch fixes those concerns by replacing the Audio
implementation with a ```NotImplemented``` version and also removes
OpenAL/NVorbis/XAudio binaries/nugets dependencies.