Update to Cortana for Android beta allows the app to become the default assistant on Android devices
There is no word on when the final version of Cortana will be ready to move out of beta, and make its way to the Google Play Store where it can be downloaded and installed with a single tap. Those interested in the beta program can click on the sourcelink.
Keep in mind that Android users will be able to scream "Hey Cortana!" at the top of their lungs without any response coming from the ol' gal. That's because of the difference in hardware between a Windows Phone and an Android handset. But for the most part, many of the features that Windows Phone users love about Cortana will be available to Android users once the beta testing ends.
source: Microsoft, WMPowerusers