re #15915 HeadlessJsTaskService is expected to run on UI thread

Summary:
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Headless tasks are required to run in the main thread, however due to the nature of the React context creation flow, the handler may be returned outside of the main thread, causing the HeadlessJsTaskContext to throw an exception.

Swipe out the app. send push notification from a server that starts a HeadlessJsTaskService
Closes #15940

Differential Revision: D5852448

Pulled By: foghina

fbshipit-source-id: 54c58a1eb7434dd5de5c39c28f6e068ed15ead9d

ReactAndroid/src/main/java/com/facebook/react/HeadlessJsTaskService.java

@@ -116,11 +116,19 @@ public void onReactContextInitialized(ReactContext reactContext) {
     }
   }
 
-  private void invokeStartTask(ReactContext reactContext, HeadlessJsTaskConfig taskConfig) {
-    HeadlessJsTaskContext headlessJsTaskContext = HeadlessJsTaskContext.getInstance(reactContext);
+  private void invokeStartTask(ReactContext reactContext, final HeadlessJsTaskConfig taskConfig) {
+    final HeadlessJsTaskContext headlessJsTaskContext = HeadlessJsTaskContext.getInstance(reactContext);
     headlessJsTaskContext.addTaskEventListener(this);
-    int taskId = headlessJsTaskContext.startTask(taskConfig);
-    mActiveTasks.add(taskId);
+
+    UiThreadUtil.runOnUiThread(
+      new Runnable() {
+        @Override
+        public void run() {
+          int taskId = headlessJsTaskContext.startTask(taskConfig);
+          mActiveTasks.add(taskId);
+        }
+      }
+    );
   }
 
   @Override