Fix android song end detection

_androidPlayer is static but wasn't being correctly treated like it was, so the listener wouldn't work correctly.

Also fix Duration and Position properties (milliseconds not seconds)
And remove all the _androidPlayer != null calls as it can never be null.

There are a few whitespace changes, so add ?w=1 to compare

Note that Android still has its own Song.cs file, I could merge this in to the shared one and send a PR for that instead if you want, that's probably the right thing to do.