FMOD resets AVAudioSession category to Ambient during background transition

Steps to reproduce:

1. Follow the steps described in this Unity discussion post: https://discussions.unity.com/t/fmod-resets-avaudiosession-category-to-ambient-during-b[…]n-6000-0-73f1-breaking-background-audio-playback/1721176/3 .
2. The key is that you need to some 3rd party native audio code (not using Unity audio) that plays looping audio via AVAudioSession / CoreAudio.
3. Start the app. Listen to the 3rd party code's audio playing.
4. Put the Unity app into the background.

Actual results: 

The audio stops playing when the app is put into the background.

Expected results: 

The audio should continue playing, if possible.

Reproducible with versions: 

6000.0.73f1

Not reproducible with versions: 

6000.0.55f1

Can’t test with versions: 

Tested on (OS): 

iOS.
Notes:

• I wouldn't really consider this a regression, because this isn't a Unity audio bug, but we try to be as non-destructive as possible for all of the 3rd party libraries and apps that work with us, so we should try to fix this.