First tinkering with Android Things on a Raspberry Pi. Demo video: https://goo.gl/photos/R6oZmnqS8XBmTNx58

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <Button
        android:id="@+id/start"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start"/>

    <TextView
        android:id="@+id/primes"
        style="?android:textAppearanceLarge"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

    <Button
        android:id="@+id/stop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Stop"/>

</LinearLayout>

app_build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 24
    buildToolsVersion '25.0.2'

    defaultConfig {
        applicationId "com.example.androidthings.myproject"
        minSdkVersion 24
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    // This is the only really interesting thing
    provided 'com.google.android.things:androidthings:0.1-devpreview'
}

MainActivity.java

package com.example.androidthings.myproject;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

import com.google.android.things.pio.Gpio;
import com.google.android.things.pio.PeripheralManagerService;

import java.io.IOException;
import java.util.Arrays;
import java.util.List;

// There are lots of better ways of doing this...but it works...
public class MainActivity extends Activity {
    private static final String TAG = MainActivity.class.getSimpleName();

    private static final String GPIO_NAME = "BCM17";

    private TextView primeStatusView;
    private Button startView;
    private Button stopView;
    private Gpio gpio17;

    private AsyncTask<Void, Integer, Void> task;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        primeStatusView = (TextView) findViewById(R.id.primes);
        startView = (Button) findViewById(R.id.start);
        stopView = (Button) findViewById(R.id.stop);

        PeripheralManagerService manager = new PeripheralManagerService();
        try {
            gpio17 = manager.openGpio(GPIO_NAME);

            gpio17.setDirection(Gpio.DIRECTION_OUT_INITIALLY_HIGH);
            gpio17.setActiveType(Gpio.ACTIVE_LOW);
        } catch (IOException e) {
            Log.w(TAG, "Unable to access GPIO", e);
        }

        startView.setEnabled(true);
        stopView.setEnabled(false);

        startView.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                showPrimes();
            }
        });

        stopView.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                primeStatusView.setText("");
                try {
                    gpio17.setValue(false);
                } catch (IOException e) {
                    Log.e(TAG, "Damn it!", e);
                }
                task.cancel(true);

                startView.setEnabled(true);
                stopView.setEnabled(false);
            }
        });
    }

    private void showPrimes() {
        startView.setEnabled(false);
        stopView.setEnabled(true);
        if (task != null) {
            task.cancel(true);
        }

        task = new AsyncTask<Void, Integer, Void>() {
            @Override
            protected Void doInBackground(Void... params) {
                List<Integer> primes = Arrays.asList(2,3,5,7,11,13,17,19,23,29);
                try {
                    gpio17.setValue(false);
                    for (Integer prime : primes) {
                        publishProgress(prime);
                        for (int i = 0; i < prime; i++) {
                            Thread.sleep(250);
                            gpio17.setValue(true);
                            Thread.sleep(250);
                            gpio17.setValue(false);
                        }

                        gpio17.setValue(false);
                        Thread.sleep(1000);
                    }
                } catch (InterruptedException ex) {
                    return null;
                }
                catch (Exception ex) {
                    Log.e(TAG, "Damn it!", ex);
                }
                return null;
            }

            @Override
            protected void onProgressUpdate(Integer... values) {
                primeStatusView.setText(primeStatusView.getText().toString() + " " + values[0].toString().trim());
            }

            @Override
            protected void onPostExecute(Void aVoid) {
                startView.setEnabled(true);
                stopView.setEnabled(false);
            }
        }.execute();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        task.cancel(true);

        if (gpio17 != null) {
            try {
                gpio17.close();
                gpio17 = null;
            } catch (IOException e) {
                Log.w(TAG, "Unable to close GPIO", e);
            }
        }
    }
}