Use Handler instead of ExoPlayer messages in ConcatenatingMediaSource

ExoPlayer methods must not be called from any thread besides the specified
app thread. Therefore we shouldn't use them here. Using a regular Handler
instead is fully equivalent.

Issue:#5240
PiperOrigin-RevId: 227650489

RELEASENOTES.md

@@ -29,6 +29,9 @@
   ([#3735](https://github.com/google/ExoPlayer/issues/3735)).
 * IMA extension: Clear ads loader listeners on release
   ([#4114](https://github.com/google/ExoPlayer/issues/4114)).
+* Fix issue where sending callbacks for playlist changes may cause problems
+  because of parallel player access
+  ([#5240](https://github.com/google/ExoPlayer/issues/5240)).
 
 ### 2.9.3 ###