Android applications seem to support some form of integration with the Android OS to identify as a music player.
Going to: Settings > Apps > *Gear Icon* > *Default Section* > Music app:
I'm able to select which application on my phone should be the default app for music, and the phone presents me with a list of choices:
Google Play Music
MX Player Pro
ES File Explorer Pro
As you can see, Spotify is not on the list. If those apps can make it to the list, so can Spotify. Would be really great to see official integration with Android in this regard, especially because such integration helps a lot when connecting the phone with the car!
Phone: OnePlus 5
OS: Oxygen OS 4.5.2 / Android 7.1.1
OS Build: ONEPLUS A5000_23_170624
(P.S. Originally posted in the Android section, but was recommended to post it here)