Make AudioTrackBufferSizeProvider public.

Was getting "java.lang.IllegalAccessError: Illegal class access" in debug app.

PiperOrigin-RevId: 465562541
(cherry picked from commit c09b34a6)

library/core/src/main/java/com/google/android/exoplayer2/audio/DefaultAudioSink.java

@@ -225,7 +225,7 @@ public final class DefaultAudioSink implements AudioSink {
   }
 
   /** Provides the buffer size to use when creating an {@link AudioTrack}. */
-  interface AudioTrackBufferSizeProvider {
+  public interface AudioTrackBufferSizeProvider {
     /** Default instance. */
     AudioTrackBufferSizeProvider DEFAULT =
         new DefaultAudioTrackBufferSizeProvider.Builder().build();