Deflake the DefaultPreloadManagerTest

From [ the last change in `DefaultPreloadManagerTest`](2b54b1e), the preloadManager began to use a separate `preloadThread` in `release_returnZeroCount_sourcesAndRendererCapabilitiesListReleased`, which unveils a bug in `PreloadMediaSource`. When `PreloadMediaSource.releasePreloadMediaSource` is called, `preloadHandler` will post a `Runnable` on the preload looper to release the internal resources. Before this `Runnable` is executed, it is possible that the [`stopPreloading`](https://github.com/androidx/media/blob/main/libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/preload/PreloadMediaSource.java#L442) method is executed just as the result of preloading has completed. This is expected to remove the posted `Runnable`s for further preloading, however, the posted `Runnable` for releasing will also be removed from the message queue.

Ideally we should use `postDelayed(runnable, token, delayMillis)` to post the runnables so that the token will be useful to identify which messages to remove in `removeCallbacksAndMessages(token)`, but that `postDelayed` method is only available from API 28. So in this change we are using a separate handler for releasing, and then the call of `preloadHandler.removeCallbacksAndMessages` won't impact the runnable for releasing.

#cherrypick

PiperOrigin-RevId: 696894483
(cherry picked from commit 0143884)

Author: tianyif

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/preload/PreloadMediaSource.java

@@ -221,6 +221,7 @@ public PreloadMediaSource createMediaSource(MediaSource mediaSource) {
   private final RendererCapabilities[] rendererCapabilities;
   private final Allocator allocator;
   private final Handler preloadHandler;
+  private final Handler releaseHandler;
   private boolean preloadCalled;
   private boolean prepareChildSourceCalled;
   private long startPositionUs;
@@ -246,6 +247,7 @@ private PreloadMediaSource(
     this.allocator = allocator;
 
     preloadHandler = Util.createHandler(preloadLooper, /* callback= */ null);
+    releaseHandler = Util.createHandler(preloadLooper, /* callback= */ null);
     startPositionUs = C.TIME_UNSET;
   }
 
@@ -396,7 +398,7 @@ protected void releaseSourceInternal() {
    * Can be called from any thread.
    */
   public void releasePreloadMediaSource() {
-    preloadHandler.post(
+    releaseHandler.post(
         () -> {
           preloadCalled = false;
           startPositionUs = C.TIME_UNSET;
@@ -407,6 +409,7 @@ public void releasePreloadMediaSource() {
           }
           releaseSourceInternal();
           preloadHandler.removeCallbacksAndMessages(null);
+          releaseHandler.removeCallbacksAndMessages(null);
         });
   }
 

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/preload/DefaultPreloadManagerTest.java

@@ -830,8 +830,7 @@ protected void releaseSourceInternal() {
   }
 
   @Test
-  public void release_returnZeroCount_sourcesAndRendererCapabilitiesListReleased()
-      throws Exception {
+  public void release_returnZeroCount_sourcesAndRendererCapabilitiesListReleased() {
     TargetPreloadStatusControl<Integer> targetPreloadStatusControl =
         rankingData -> new DefaultPreloadManager.Status(STAGE_SOURCE_PREPARED);
     MediaSource.Factory mockMediaSourceFactory = mock(MediaSource.Factory.class);