Split MediaPlayer/Song on a per-API basis

This PR is similar to https://github.com/mono/MonoGame/pull/2251 and https://github.com/mono/MonoGame/pull/2270

The goal being to help clean up the multiple implementations of MediaPlayer/Song by merging the multiple source files into a single partial class. This'll allow for platforms to easily be added later, as well as share common code between them. (null checks, variable setting, etc). 

This PR doesn't contain any functional changes.

NOTE: This was branched from https://github.com/mono/MonoGame/pull/2275 Merge this PR after that one's been closed out.