Improve housekeeping of ConcatenatingMediaSource callbacks.

When calling releaseSource(), all pending messages will be removed. That means
that all action-on-completion callbacks which are somewhere in flight are
just dropped without being called. This change adds code to keep track of the
current state of each callback to allow all of them being called when the
source is released.

Issue:#5464
PiperOrigin-RevId: 232312528

RELEASENOTES.md

@@ -14,6 +14,8 @@
 * OkHttp extension: Upgrade OkHttp dependency to 3.12.1.
 * MP3: Wider fix for issue where streams would play twice on some Samsung
   devices ([#4519](https://github.com/google/ExoPlayer/issues/4519)).
+* Fix issue with dropped messages when releasing a `ConcatenatingMediaSource`
+  ([#5464](https://github.com/google/ExoPlayer/issues/5464)).
 
 ### 2.9.4 ###