Publish ConcatenatingMediaSource2

Can be used to combine multiple media items into a single timeline window.

Issue: androidx/media#247
Issue: google/ExoPlayer#4868
PiperOrigin-RevId: 506283307
(cherry picked from commit fcd3af64)

RELEASENOTES.md

@@ -17,6 +17,9 @@
         for seeking.
     *   Use theme when loading drawables on API 21+
         ([#220](https://github.com/androidx/media/issues/220)).
+    *   Add `ConcatenatingMediaSource2` that allows combining multiple media
+        items into a single window
+        ([#247](https://github.com/androidx/media/issues/247)).
 *   Extractors:
     *   Throw a ParserException instead of a NullPointerException if the sample
         table (stbl) is missing a required sample description (stsd) when