The 'OK Google' command continues to expand. The Moto X made it possible to activate Google services with a simple voice command, and a similar feature is now coming to most Android devices. Google Search 3.5.14 brings with it an expansion of OK Google. Users with the latest version of Search will be able to activate Search from the lock screen, homescreen, and from within apps.
Once you have the latest version of Search installed, you will have to enable universal hotword detection from within the settings.