Fix Fmp4Extractor.init to use text transcoding ExtractorOutput

This was missed in da724c8cc4

I tried to write a test for this, but got stuck crafting valid test
data. I was able to create a new fragmented MP4 file containing only a
TTML track:

```shell
$ MP4Box -add simple.ttml -frag 2000 sample_fragmented_ttml.mp4
```

Then I tried naively removing the `ftyp` and `moov` boxes with a hex
editor, but using this in `FragmentedMp4ExtractorNoSniffingTest` gave
me an `EOFException` that I didn't get to the root cause of.

Issue: androidx/media#1779

#cherrypick

PiperOrigin-RevId: 683667850

RELEASENOTES.md

@@ -128,6 +128,9 @@
 *   HLS Extension:
 *   DASH Extension:
 *   Smooth Streaming Extension:
+    *   Fix a `Bad magic number for Bundle` error when playing SmoothStreaming
+        streams with text tracks
+        ([#1779](https://github.com/androidx/media/issues/1779)).
 *   RTSP Extension:
     *   Fix user info removal for URLs that contain encoded @ characters
         ([#1138](https://github.com/androidx/media/pull/1138)).

libraries/extractor/src/main/java/androidx/media3/extractor/mp4/FragmentedMp4Extractor.java

@@ -448,7 +448,7 @@ public class FragmentedMp4Extractor implements Extractor {
     if (sideloadedTrack != null) {
       TrackBundle bundle =
           new TrackBundle(
-              output.track(0, sideloadedTrack.type),
+              extractorOutput.track(0, sideloadedTrack.type),
               new TrackSampleTable(
                   sideloadedTrack,
                   /* offsets= */ new long[0],