It would appear that Google is testing a button for Android Auto that will disconnect a smartphone. It will be in the area of your smartphone designated for notifications, and tapping it will sever the connection that your phone has established with the wireless Android Auto.
Android Auto 10.6 Update
The function is apparently being made available with the Android Auto 10.6 update that can be downloaded from the Play Store; however, it appears that only a few beta testers have had it appear.
This is a very useful feature considering that, up until now, users were required to either put their phone into airplane mode or physically disconnect an adapter in order to disconnect it from Android Auto.
People who want to switch between two Android phones while they are driving will also find it convenient to use this feature.