Fix flag issue causing corrupt output on DRM playbacks.

The issue is visible for DRM playbacks because
BUFFER_FLAG_ENCRYPTED is being cleared, which results
in trying to play encrypted samples without decryption.
The issue would have also incorrectly cleared key frame
flags too, for both clear and encrypted playbacks, but
I don't think we're using that flag downstream anywhere
and so the issue wasn't visible.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123426582

library/src/main/java/com/google/android/exoplayer/extractor/DefaultTrackOutput.java

@@ -239,7 +239,7 @@ public final class DefaultTrackOutput implements TrackOutput {
         return TrackStream.FORMAT_READ;
       case TrackStream.BUFFER_READ:
         if (buffer.timeUs < decodeOnlyUntilUs) {
-          buffer.setFlags(C.BUFFER_FLAG_DECODE_ONLY);
+          buffer.addFlag(C.BUFFER_FLAG_DECODE_ONLY);
         }
         // Read encryption data if the sample is encrypted.
         if (buffer.isEncrypted()) {