Respect decode-only flag in MetadataRenderer

Issue #2176

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144999973

library/src/main/java/com/google/android/exoplayer2/metadata/MetadataRenderer.java

@@ -113,6 +113,10 @@ public void render(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackEx
       if (result == C.RESULT_BUFFER_READ) {
         if (buffer.isEndOfStream()) {
           inputStreamEnded = true;
+        } else if (buffer.isDecodeOnly()) {
+          // Do nothing. Note this assumes that all metadata buffers can be decoded independently.
+          // If we ever need to support a metadata format where this is not the case, we'll need to
+          // pass the buffer to the decoder and discard the output.
         } else {
           pendingMetadataTimestamp = buffer.timeUs;
           buffer.subsampleOffsetUs = formatHolder.format.subsampleOffsetUs;