Make VpxOutputBuffer public

Mimics the ExoV1 behavior.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217893212

extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxOutputBuffer.java

@@ -19,10 +19,8 @@ import com.google.android.exoplayer2.decoder.OutputBuffer;
 import com.google.android.exoplayer2.video.ColorInfo;
 import java.nio.ByteBuffer;
 
-/**
- * Output buffer containing video frame data, populated by {@link VpxDecoder}.
- */
-/* package */ final class VpxOutputBuffer extends OutputBuffer {
+/** Output buffer containing video frame data, populated by {@link VpxDecoder}. */
+public final class VpxOutputBuffer extends OutputBuffer {
 
   public static final int COLORSPACE_UNKNOWN = 0;
   public static final int COLORSPACE_BT601 = 1;