[Feature request] Ability to play video/audio on multiple VideoViews simultaneously

Adding the ability to play video/audio in multiple VideoViews simultaneously.

The current restriction of only one VideoView can play is due to the referencing to the singleton CrossMediaManager.Current below:

```
///
/// Cross platform MediaManager implemenations
///
public class CrossMediaManager
{
static readonly Lazy Implementation = new Lazy(CreateMediaManager);

///
/// Current settings to use
///
public static IMediaManager Current
{
get
{

if (Implementation.Value == null)
{
throw NotImplementedInReferenceAssembly();
}
return Implementation.Value;
}
}
```