Fix triggering messages sent from non-Looper threads.

This can happen for instrumented tests that are run on a non-Looper
thread. If these tests send a message to a Looper thread to start the
test procedure, they should just triger the message directly as before.

PiperOrigin-RevId: 354066836

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeClock.java

@@ -149,12 +149,18 @@ public class FakeClock implements Clock {
   protected synchronized void addPendingHandlerMessage(HandlerMessage message) {
   protected synchronized void addPendingHandlerMessage(HandlerMessage message) {
     handlerMessages.add(message);
     handlerMessages.add(message);
     if (!waitingForMessage) {
     if (!waitingForMessage) {
-      // If this isn't executed from inside a message created by this class, make sure the current
-      // looper message is finished before handling the new message.
+      // This method isn't executed from inside a looper message created by this class.
+      @Nullable Looper currentLooper = Looper.myLooper();
+      if (currentLooper == null) {
+        // This message is triggered from a non-looper thread, so just execute it directly.
+        maybeTriggerMessage();
+      } else {
+        // Make sure the current looper message is finished before handling the new message.
         waitingForMessage = true;
         waitingForMessage = true;
         new Handler(checkNotNull(Looper.myLooper())).post(this::onMessageHandled);
         new Handler(checkNotNull(Looper.myLooper())).post(this::onMessageHandled);
       }
       }
     }
     }
+  }
 
 
   private synchronized void removePendingHandlerMessages(ClockHandler handler, int what) {
   private synchronized void removePendingHandlerMessages(ClockHandler handler, int what) {
     for (int i = handlerMessages.size() - 1; i >= 0; i--) {
     for (int i = handlerMessages.size() - 1; i >= 0; i--) {