Handle getting the audio track's position before the first AC-3 buffer.

ac3Bitrate is set only after the first buffer is handled, which meant that
getting the playback position would cause a divide by zero before then.

When playing back AC-3 content, the ac3Bitrate will always be set after the
first buffer is handled, so return a 0 position if it is not set.

library/src/main/java/com/google/android/exoplayer/audio/AudioTrack.java

@@ -641,7 +641,8 @@ public final class AudioTrack {
 
   private long bytesToFrames(long byteCount) {
     if (isAc3) {
-      return byteCount * 8 * sampleRate / (1000 * ac3Bitrate);
+      return
+          ac3Bitrate == UNKNOWN_AC3_BITRATE ? 0L : byteCount * 8 * sampleRate / (1000 * ac3Bitrate);
     } else {
       return byteCount / frameSize;
     }