Provide example of android side implementation of HeadlessJS

Summary:
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The existing documentation on this topic doesn't provide enough of detail on where to start a headlessJS task.

Just docs change

This PR extends HeadlessJS documentation to show an example on how to start HeadlessJsTaskService. Sample code in example reacts to connectivity change using custom broadcast receiver logic that bundles additional information and starts the aforementioned service.
Closes https://github.com/facebook/react-native/pull/13957

Differential Revision: D5746809

Pulled By: hramos

fbshipit-source-id: 3eeea7d4a71382acf4b6f7ad1b99d20e2745c558

docs/HeadlessJSAndroid.md

@@ -75,4 +75,74 @@ getApplicationContext().startService(service);
 * By default, your app will crash if you try to run a task while the app is in the foreground. This is to prevent developers from shooting themselves in the foot by doing a lot of work in a task and slowing the UI. You can pass a fourth `boolean` argument to control this behaviour.
 * If you start your service from a `BroadcastReceiver`, make sure to call `HeadlessJsTaskService.acquireWakeLockNow()` before returning from `onReceive()`.
 
+## Example Usage
+
+Service can be started from Java API. First you need to decide when the service should be started and implement your solution accordingly. Here is a simple example that reacts to network connection change.
+
+Following lines shows part of Android manifest file for registering broadcast receiver.   
+```xml
+<receiver android:name=".NetworkChangeReceiver" >
+  <intent-filter>
+    <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
+  </intent-filter>
+</receiver>
+```
+
+Broadcast receiver then handles intent that was broadcasted in onReceive function. This is a great place to check whether your app is on foreground or not. If app is not on foreground we can prepare our intent to be started, with no information or additional information bundled using putExta (keep in mind bundle can handle only parcelable values). In the end service is started and wakelock is acquired.
+
+```java
+public class NetworkChangeReceiver extends BroadcastReceiver {
+
+    @Override
+    public void onReceive(final Context context, final Intent intent) {
+        /**
+          This part will be called everytime network connection is changed
+          e.g. Connected -> Not Connected
+        **/
+        if (!isAppOnForeground((context))) {
+            /**
+              We will start our service and send extra info about 
+              network connections
+            **/
+            boolean hasInternet = isNetworkAvailable(context);
+            Intent serviceIntent = new Intent(context, MyTaskService.class);
+            serviceIntent.putExtra("hasInternet", hasInternet);
+            context.startService(serviceIntent);
+            HeadlessJsTaskService.acquireWakeLockNow(context);
+        }
+    }
+
+    private boolean isAppOnForeground(Context context) {
+        /**
+          We need to check if app is in foreground otherwise the app will crash.
+         http://stackoverflow.com/questions/8489993/check-android-application-is-in-foreground-or-not
+        **/
+        ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
+        List<ActivityManager.RunningAppProcessInfo> appProcesses = 
+        activityManager.getRunningAppProcesses();
+        if (appProcesses == null) {
+            return false;
+        }
+        final String packageName = context.getPackageName();
+        for (ActivityManager.RunningAppProcessInfo appProcess : appProcesses) {
+            if (appProcess.importance == 
+            ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND &&
+             appProcess.processName.equals(packageName)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public static boolean isNetworkAvailable(Context context) {
+        ConnectivityManager cm = (ConnectivityManager) 
+        context.getSystemService(Context.CONNECTIVITY_SERVICE);
+        NetworkInfo netInfo = cm.getActiveNetworkInfo();
+        return (netInfo != null && netInfo.isConnected());
+    }
+
+
+}
+```
+
 [0]: https://developer.android.com/reference/android/content/Context.html#startService(android.content.Intent)