Use Handler to post action schedule finished callback.

Calling it directly might skip other callbacks. For example:
ActionSchedule.Builder().waitForTimelineChanged(...).build().
is currently immediately calling through to
callback.onActionScheduleFinished when the timeline changes.
Depending on the position of the action schedule listener in the
listener set, it may skip other listeners also
listening to timeline changes.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177442975

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

@@ -41,6 +41,7 @@ import com.google.android.exoplayer2.testutil.Action.WaitForPositionDiscontinuit
 import com.google.android.exoplayer2.testutil.Action.WaitForSeekProcessed;
 import com.google.android.exoplayer2.testutil.Action.WaitForSeekProcessed;
 import com.google.android.exoplayer2.testutil.Action.WaitForTimelineChanged;
 import com.google.android.exoplayer2.testutil.Action.WaitForTimelineChanged;
 import com.google.android.exoplayer2.trackselection.MappingTrackSelector;
 import com.google.android.exoplayer2.trackselection.MappingTrackSelector;
+import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.Clock;
 import com.google.android.exoplayer2.util.Clock;
 
 
 /**
 /**
@@ -487,13 +488,30 @@ public final class ActionSchedule {
     }
     }
 
 
     @Override
     @Override
-    protected void doActionImpl(SimpleExoPlayer player, MappingTrackSelector trackSelector,
-        Surface surface) {
+    protected void doActionAndScheduleNextImpl(
+        SimpleExoPlayer player,
+        MappingTrackSelector trackSelector,
+        Surface surface,
+        Handler handler,
+        ActionNode nextAction) {
+      Assertions.checkArgument(nextAction == null);
       if (callback != null) {
       if (callback != null) {
-        callback.onActionScheduleFinished();
+        handler.post(
+            new Runnable() {
+              @Override
+              public void run() {
+                callback.onActionScheduleFinished();
+              }
+            });
       }
       }
     }
     }
 
 
+    @Override
+    protected void doActionImpl(
+        SimpleExoPlayer player, MappingTrackSelector trackSelector, Surface surface) {
+      // Not triggered.
+    }
+
   }
   }
 
 
 }
 }