Fix race condition in DownloadHelper

Sending MESSAGE_PREPARE_SOURCE should happen last in the constructor.
It was previously happening before initialization finished (and in
particular before pendingMediaPeriods was instantiated).

Issue: #6146
PiperOrigin-RevId: 257158275

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java

@@ -809,10 +809,10 @@ public final class DownloadHelper {
     private final MediaSource mediaSource;
     private final DownloadHelper downloadHelper;
     private final Allocator allocator;
+    private final ArrayList<MediaPeriod> pendingMediaPeriods;
+    private final Handler downloadHelperHandler;
     private final HandlerThread mediaSourceThread;
     private final Handler mediaSourceHandler;
-    private final Handler downloadHelperHandler;
-    private final ArrayList<MediaPeriod> pendingMediaPeriods;
 
     @Nullable public Object manifest;
     public @MonotonicNonNull Timeline timeline;
@@ -824,6 +824,7 @@ public final class DownloadHelper {
       this.mediaSource = mediaSource;
       this.downloadHelper = downloadHelper;
       allocator = new DefaultAllocator(true, C.DEFAULT_BUFFER_SEGMENT_SIZE);
+      pendingMediaPeriods = new ArrayList<>();
       @SuppressWarnings("methodref.receiver.bound.invalid")
       Handler downloadThreadHandler = Util.createHandler(this::handleDownloadHelperCallbackMessage);
       this.downloadHelperHandler = downloadThreadHandler;
@@ -831,7 +832,6 @@ public final class DownloadHelper {
       mediaSourceThread.start();
       mediaSourceHandler = Util.createHandler(mediaSourceThread.getLooper(), /* callback= */ this);
       mediaSourceHandler.sendEmptyMessage(MESSAGE_PREPARE_SOURCE);
-      pendingMediaPeriods = new ArrayList<>();
     }
 
     public void release() {