Set lock screen background in Android (like Spotify do)

Edit:
The solution below only works for applications that have registered itself as a media controller, so apps that don’t play audio can’t/shouldn’t use this mechanism to change the lockscreen wallpaper.

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It can be done using RemoteControlClient, part of Android since ICS. If you want a working example, download VLC for Android and check out org.videolan.vlc.AudioService:

This part of the code is to intercept media controls.

/**
 * Set up the remote control and tell the system we want to be the default receiver for the MEDIA buttons
 * @see http://android-developers.blogspot.fr/2010/06/allowing-applications-to-play-nicer.html
 */
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
public void setUpRemoteControlClient() {
    Context context = VLCApplication.getAppContext();
    AudioManager audioManager = (AudioManager)context.getSystemService(AUDIO_SERVICE);

    if(Util.isICSOrLater()) {
        audioManager.registerMediaButtonEventReceiver(mRemoteControlClientReceiverComponent);

        if (mRemoteControlClient == null) {
            Intent mediaButtonIntent = new Intent(Intent.ACTION_MEDIA_BUTTON);
            mediaButtonIntent.setComponent(mRemoteControlClientReceiverComponent);
            PendingIntent mediaPendingIntent = PendingIntent.getBroadcast(context, 0, mediaButtonIntent, 0);

            // create and register the remote control client
            mRemoteControlClient = new RemoteControlClient(mediaPendingIntent);
            audioManager.registerRemoteControlClient(mRemoteControlClient);
        }

        mRemoteControlClient.setTransportControlFlags(
                RemoteControlClient.FLAG_KEY_MEDIA_PLAY |
                RemoteControlClient.FLAG_KEY_MEDIA_PAUSE |
                RemoteControlClient.FLAG_KEY_MEDIA_PREVIOUS |
                RemoteControlClient.FLAG_KEY_MEDIA_NEXT |
                RemoteControlClient.FLAG_KEY_MEDIA_STOP);
    } else if (Util.isFroyoOrLater()) {
        audioManager.registerMediaButtonEventReceiver(mRemoteControlClientReceiverComponent);
    }
}

This part is to update artwork, among other info:

@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
private void updateRemoteControlClientMetadata() {
    if(!Util.isICSOrLater()) // NOP check
        return;

    if (mRemoteControlClient != null) {
        MetadataEditor editor = mRemoteControlClient.editMetadata(true);
        editor.putString(MediaMetadataRetriever.METADATA_KEY_ALBUM, getCurrentMedia().getAlbum());
        editor.putString(MediaMetadataRetriever.METADATA_KEY_ARTIST, getCurrentMedia().getArtist());
        editor.putString(MediaMetadataRetriever.METADATA_KEY_GENRE, getCurrentMedia().getGenre());
        editor.putString(MediaMetadataRetriever.METADATA_KEY_TITLE, getCurrentMedia().getTitle());
        editor.putLong(MediaMetadataRetriever.METADATA_KEY_DURATION, getCurrentMedia().getLength());
        editor.putBitmap(MetadataEditor.BITMAP_KEY_ARTWORK, getCover());
        editor.apply();
    }
}