Run custom messages executed on playback thread immediately.

This ensures message order if multiple custom messages running on the
playback thread and direct player commands are called immedately after
each other.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179925852

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -904,13 +904,17 @@ import java.util.Collections;
             }
           }
         };
-    handler.post(
-        new Runnable() {
-          @Override
-          public void run() {
-            customMessageInfo.message.getHandler().post(handleMessageRunnable);
-          }
-        });
+    if (customMessageInfo.message.getHandler().getLooper() == handler.getLooper()) {
+      handleMessageRunnable.run();
+    } else {
+      handler.post(
+          new Runnable() {
+            @Override
+            public void run() {
+              customMessageInfo.message.getHandler().post(handleMessageRunnable);
+            }
+          });
+    }
   }
 
   private void resolveCustomMessagePositions() {