Ok Google hotword and voice actions support is very limited in Android right now. Apart from Moto X, there is no other Android device that supports the voice-commands outside of Google voice search or home-screen (in Google Now launcher). If sources of Android Police are to be believed, Google is planning to change this by bringing Ok Google hotword and voice-actions support across the Android operating system and even inside the applications (at least some).
According to the blog, Google is currently working to integrate voice-commands in Android, which will work whether or not you are in Search app or on home-screen. You could be inside Calendar app and just say “Ok Google, add an event” and the operating system and app will understand you and will create a new calendar entry. There is no word on whether such voice actions will be supported in all apps or just Google applications. It is possible that Google will start with its own apps and provide some API and other tools to the developers to integrate the same functionality in their own apps.
Among other changes, Google is also reportedly working on a new navigation buttons setup, which will replace home-button with Google button, which will be linked to Google Search. Android Police notes that this particular set-up might only be implanted in Nexus and Google Play edition devices that come pre-loaded with Google Now launcher and not in devices from other manufacturers as they use their own skins and button set-up.
There is no word on when these changes might be released. There is also a possibility that Google might scrap them altogether as the companies always keep working on new ideas, some of which are discarded due to various reasons.
Source: Android Police