stream: Preserve enum class type in GetState()

Preserves the meaning/type-safetiness of the stream state instead of
making it an opaque u32. This makes it usable for other things outside
of the service HLE context.

src/audio_core/audio_renderer.cpp

@@ -79,7 +79,7 @@ u32 AudioRenderer::GetMixBufferCount() const {
     return worker_params.mix_buffer_count;
 }
 
-u32 AudioRenderer::GetState() const {
+Stream::State AudioRenderer::GetStreamState() const {
     return stream->GetState();
 }