Android allows to choose the audio codec that should be used when an audio device like the Fiio BTR15 is being connect via Bluetooth. I have set that to LDAC and it stays like that.
However, Android always chooses 96kHz and 24bit, which is not only a waste of bandwith but also needs resampling 100% of my music and most of files of most other people, as the vast majority of digital music will be in 44,1kHz and 16bit. Resampling and waste of bandwidth both mean an unnecessary loss in quality. You can change it to 44,1kHz and 16bit or 24bit in the delevoper options, but Android always defaults back to 96/24 when a device is reconnected. Can changed settings be made permanent, somehow?