core_timing: Convert core timing into a class

Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.

Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.

src/audio_core/audio_out.cpp

@@ -26,14 +26,15 @@ static Stream::Format ChannelsToStreamFormat(u32 num_channels) {
     return {};
 }
 
-StreamPtr AudioOut::OpenStream(u32 sample_rate, u32 num_channels, std::string&& name,
+StreamPtr AudioOut::OpenStream(Core::Timing::CoreTiming& core_timing, u32 sample_rate,
+                               u32 num_channels, std::string&& name,
                                Stream::ReleaseCallback&& release_callback) {
     if (!sink) {
         sink = CreateSinkFromID(Settings::values.sink_id, Settings::values.audio_device_id);
     }
 
     return std::make_shared<Stream>(
-        sample_rate, ChannelsToStreamFormat(num_channels), std::move(release_callback),
+        core_timing, sample_rate, ChannelsToStreamFormat(num_channels), std::move(release_callback),
         sink->AcquireSinkStream(sample_rate, num_channels, name), std::move(name));
 }