entrypoint of android application

The first "entry" point is the application class as Kingston pointed out.

However, the easiest thing to get the very first starting point is to check the stack when debugging onCreate.

You may check Instrumentation, this sound somewhat like what you want.

http://developer.android.com/reference/android/app/Instrumentation.html

MainActivity.onCreate(Bundle) line: 12  
Instrumentation.callActivityOnCreate(Activity, Bundle) line: 1047   
ActivityThread.performLaunchActivity(ActivityThread$ActivityRecord, Intent) line: 2627  
ActivityThread.handleLaunchActivity(ActivityThread$ActivityRecord, Intent) line: 2679   
ActivityThread.access$2300(ActivityThread, ActivityThread$ActivityRecord, Intent) line: 125 
ActivityThread$H.handleMessage(Message) line: 2033  
ActivityThread$H(Handler).dispatchMessage(Message) line: 99 
Looper.loop() line: 123 
ActivityThread.main(String[]) line: 4627    
Method.invokeNative(Object, Object[], Class, Class[], Class, int, boolean) line: not available [native method]  
Method.invoke(Object, Object...) line: 521  
ZygoteInit$MethodAndArgsCaller.run() line: 868  
ZygoteInit.main(String[]) line: 626 
NativeStart.main(String[]) line: not available [native method]  

You should extend the Application class and override the onCreate method.

For reference:Application class

In core Java programs we need a main() method, because while executing the byte code the JVM(Java Virtual Machine) will search for the main() method in the class and start executing there.

In Android, the (DVM)Dalvik Virtual Machine is designed to find a class which is a subclass of Activity and which is set as a LAUNCHER to start the execution of the application from its onCreate() method, so there is no need of a main() method.

I don't know it myself, but it sounds an interesting question. This is the code that fires a new Activity and following the code, you'll end up in JNI code

public void startActivityForResult(Intent intent, int requestCode) {
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }
        } else {
            mParent.startActivityFromChild(this, intent, requestCode);
        }
    }

Android source code is available, but it's a bit tricky to get it because it's poorly documented. You'll have to install repo and then download the framework/base project