一个App的程序入口到底是什么？

是ActivityThread.main()。

ActivityThread.main() 方法解析

main() 方法中

 public static void main(String[] args) {

          ...ignore some code...    

      Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        AsyncTask.init();

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        Looper.loop();

          ...ignore some code...    

 }