LibAudio: Use NonnullOwnPtr to keep track of LoaderPlugin streams

This doesn't have any immediate uses, but this adapts the code a bit
more to `Core::Stream` conventions (as most functions there use
NonnullOwnPtr to handle streams) and it makes it a bit clearer that this
pointer isn't actually supposed to be null. In fact, MP3LoaderPlugin
and FlacLoaderPlugin apparently forgot to check for that completely
before starting to decode data.

Userland/Libraries/LibAudio/Loader.cpp

@@ -11,7 +11,7 @@
 
 namespace Audio {
 
-LoaderPlugin::LoaderPlugin(OwnPtr<Core::Stream::SeekableStream> stream)
+LoaderPlugin::LoaderPlugin(NonnullOwnPtr<Core::Stream::SeekableStream> stream)
     : m_stream(move(stream))
 {
 }