External USB headphone DAC/amps for Android phones are becoming a thing. The release of the LG G5 and its "Hi-Fi Plus module" is an example that has been widely publicised. What Hi-Fi has a Best DACs page that mainly lists portable audiophile kit - a few years ago the DACs in What Hi-Fi were all mains powered and large. I would personally prefer to carry a phone and a DAC rather than a phone and a portable music player mostly because of Spotify.
AudioQuest, makers of the Dragonfly, have recommended the USB Audio Player PRO (UAPP) app. The app uses its own USB Audio Driver to bypass the imperfect Android driver. On my phone I get glitching and up-sampling in Spotify with the Dragonfly and the Android driver but there are no issues with UAPP.
Spotify developers could work with the makers of USB Audio Player PRO (UAPP) to support their USB Audio driver in Spotify. I imagine that UAPP would need to be installed before Spotify would see the driver. So I imagine UAPP's developers would help with the integration because it would lead to more sales of their app.
The UAPP developers are clearly experts at linking Android phones to USB audio devices.
If there is a concern about protecting the IP in the music then working with the developer of the USB driver could be useful. e.g. to verify that the end point was a supported device.
I expect Google will improve the Android drivers in the future but Spotify users, like me, are looking for audiophile sound quality now and on their current devices.
Spotify's competitor Tidal is already supported by UAPP.