[WP8] Calling SoundEffectInstance.Stop(true) unstable state.

By calling SoundEffectInstance.Stop(true), any subsequent calls to
SoundEffectInstance.Play(...) have no effect.

By calling Calling SoundEffectInstance.Stop(true) the object gets into
an unstable state.

The buffer is not cleared, which causes the .State to always return
.Playing , which in turn causes .Play() to always return early without
starting the _voice.

The behavior of .Stop(true) should be similar to .Stop().
clean up code

Clean up code
Merge pull request #2234 from nkast/tnc_SoundEffectInstance_Stop

[WP8] Calling SoundEffectInstance.Stop(true) unstable state.