[WP8] fix memory leak when Resume game

ContentManager & ContentReaders was creating new instances during
AssetReload. The problem was with SoundEffectReader, VertexBufferReader
and generally with Shared resources like VertexBuffer, IndexBuffer,
Effect.

The problem was previously reported here:
https://github.com/mono/MonoGame/issues/1771

http://community.monogame.net/t/sharpdx-crashing-the-app-after-having-resumed-a-few-times/813/10

Tested on WP8.