Spotify pauses for about 2 seconds whenever I receive a message from Google Hangouts. This all happens whenever I am actively using the app. What noticed is that when I turn off notification completely and I receive a message, the music continues to play. If I have notifications on, then it pauses for about two seconds. I uninstalled and reinstalled Spotify and it is still doing the same thing.
The same is true on the iphone, any notification that produces an audible alert wil interupt music. I suspect turning off the audible alert will help or maybe try the do not disturb feature that will stop alerts while listening to music.