SoundEffect/SoundEffectPool Fixes

A few fixes for unexpected issues from https://github.com/mono/MonoGame/pull/2677.

 - Fixed `SoundEffect.GetPooledInstance()` to set the `_effect`.
 - Unified the work `SoundEffect.Play()`, `SoundEffect.Play(float, float, float)`, and `SoundEffect.GetPooledInstance()` do.
 -  `SoundEffect.Play()` no longer does the extra work to set volume, pitch, and pan.
 - Removed redundant setting of `_effect.
 - `_playingInstances` and `_pooledInstances` both get default capacities now.
 - Better calculation of default pool capacity that doesn't require #ifs.
 - We now handle exhausting the sound pool in Xact.

Good point...  I totally missed that! :)