WinRT: Ignore media in MediaPlayer only if stream is not changed too

When qrc media content is set, it allocates and opens QFile.

If the same content is set again, the url remains the same,
but it creats new QFile, and need to update by new stream.

Change-Id: Iee6c50135bb8304177affbe70b27d24898fc5b5c
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>