Adds key functionality for basic MIDI synthesization

- Add a MidiRenderer skeleton with basic playback
- Add MidiDecoder with placeholder synthesizer (JSyn) impl
- Make MidiExtractor feed placeholder data
- Add Renderer extension loaders for MIDI

PiperOrigin-RevId: 448212797

library/core/src/main/java/com/google/android/exoplayer2/DefaultRenderersFactory.java

@@ -483,6 +483,19 @@ public class DefaultRenderersFactory implements RenderersFactory {
       extensionRendererIndex--;
     }
 
+    try {
+      Class<?> clazz = Class.forName("com.google.android.exoplayer2.decoder.midi.MidiRenderer");
+      Constructor<?> constructor = clazz.getConstructor();
+      Renderer renderer = (Renderer) constructor.newInstance();
+      out.add(extensionRendererIndex++, renderer);
+      Log.i(TAG, "Loaded MidiRenderer.");
+    } catch (ClassNotFoundException e) {
+      // Expected if the app was built without the extension.
+    } catch (Exception e) {
+      // The extension is present, but instantiation failed.
+      throw new RuntimeException("Error instantiating MIDI extension", e);
+    }
+
     try {
       // Full class names used for constructor args so the LINT rule triggers if any of them move.
       Class<?> clazz = Class.forName("com.google.android.exoplayer2.ext.opus.LibopusAudioRenderer");