http://www.techradar.com/news/alexa-voice-commands-are-finally-coming-to-the-android-alexa-app

Fittingly enough, support for Amazon's Alexa voice assistant is finally coming to the Alexa app for Android phones, although it's currently a bit of a gimmick.

Rather than being able to talk to Alexa at any time much as you would through Google Assistant, Alexa only works on Android devices if you have the Alexa app open, as Android Police reports. But hey, small steps, right?