FSEvent.GetUuidForDevice is incorrect

The FSEvent.GetUuidForDevice has a few problems:

https://github.com/xamarin/xamarin-macios/blob/313bc16bc76b41b29b520c1d43af03c0ea9db8d5/src/CoreServices/FSEvents.cs#L94-L106

1. `FSEventsCopyUUIDForDevice` returns a retained object reference. This value isn't released (so it's leaked).
2. `FSEventsCopyUUIDForDevice ` returns an object reference. The code treats it like a blob of memory that contains the UUID. The fix is probably to call [`CFUUIDGetUUIDByte `](https://developer.apple.com/documentation/corefoundation/1542119-cfuuidgetuuidbytes?language=objc) on the returned pointer.
3. No tests

Start by reading src/CoreServices/FSEvents.cs at lines 94-106 and the linked Apple API documentation for FSEventsCopyUUIDForDevice and CFUUIDGetUUIDByte. Add tests for the FSEvent.GetUuidForDevice behavior; done means the returned UUID is read correctly, the retained object is released, and the tests pass.

Written by the indexing model from the issue text.