An activity which copies an APK from assets to private directory, and loads a class and resources from it.

DynamicActivity.java

package whatever.dynamicapkloading;

import android.app.Activity;
import android.content.Context;
import android.content.res.AssetManager;
import android.content.res.Configuration;
import android.content.res.Resources;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.widget.TextView;
import dalvik.system.PathClassLoader;

import java.io.*;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

public class DynamicActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        File targetApk = new File(getDir("dex", Context.MODE_PRIVATE), "app.apk");
        // copy APK from assets to private directory
        if (!targetApk.exists() || !targetApk.isFile()) { // remove this condition in order to keep dynamic APK fresh
            try (BufferedInputStream bis = new BufferedInputStream(getAssets().open("dynamic-debug.apk"));
                 OutputStream dexWriter = new BufferedOutputStream(new FileOutputStream(targetApk))) {

                byte[] buf = new byte[4096];
                int len;
                while((len = bis.read(buf, 0, 4096)) > 0) {
                    dexWriter.write(buf, 0, len);
                }
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

        Runnable dynamicInstance;
        Resources dynamicResources;
        int anotherStringId;
        try {
            // load class & instantiate it
            PathClassLoader loader = new PathClassLoader(targetApk.getAbsolutePath(), getClassLoader());
            Class<?> dynamicClass = loader.loadClass("whatever.dynamic.Code");
            Constructor<?> ctor = dynamicClass.getConstructor(Context.class);
            dynamicInstance = (Runnable) ctor.newInstance(this);

            // load resources
            AssetManager assets = AssetManager.class.newInstance();
            Method addAssetPath = AssetManager.class.getMethod("addAssetPath", String.class);
            if (addAssetPath.invoke(assets, targetApk.getAbsolutePath()) == Integer.valueOf(0)) {
                throw new RuntimeException();
            }

            Class<?> resourcesImpl = Class.forName("android.content.res.ResourcesImpl");
            Class<?> daj = Class.forName("android.view.DisplayAdjustments");
            Object impl = resourcesImpl
                    .getConstructor(AssetManager.class, DisplayMetrics.class, Configuration.class, daj)
                    .newInstance(assets, getResources().getDisplayMetrics(), getResources().getConfiguration(), daj.newInstance());

            dynamicResources = Resources.class.getConstructor(ClassLoader.class).newInstance(loader);
            Method setImpl = Resources.class.getMethod("setImpl", Class.forName("android.content.res.ResourcesImpl"));
            setImpl.invoke(dynamicResources, impl);

            Class<?> rString = Class.forName("whatever.dynamic.R$string", true, loader);
            anotherStringId = rString.getField("anotherString").getInt(null);
        } catch (ReflectiveOperationException e) {
            throw new RuntimeException(e);
        }

        // create UI
        TextView textView = new TextView(this);
        setContentView(textView);

        // we're done, run!
        dynamicInstance.run();

        // grab some resources
        int someStringId = dynamicResources.getIdentifier("someString", "string", "whatever.dynamic");
        String someString = dynamicResources.getString(someStringId);

        String anotherString = dynamicResources.getString(anotherStringId);
        textView.setText(someString + " | " + anotherString);
    }
}