Android AudioAttributes can't be adjusted for SimpleAudioPlayer

[gkarabin]

I would like to for my app to be able to change the Android AudioAttributes associated with the MediaPlayer instance in SimpleAudioPlayerImplementation.cs.

I can think of two obvious approaches: either expose the underlying MediaPlayer (which has Dispose logic to account for) or add methods to allow runtime setting of the AudioAttributes. I would like to get your opinion on which approach you'd prefer, if any, for a PR.

It occurs to me that it would make sense to make similar changes for the other implementations, at which point some simplicity is lost in favor of flexibility. I could see not wanting this change at all for that reason.

What do you think?