Is there an app_config.xml on android?

I would be very surprised if there isn't. However accessing it with a text editor is likely to be difficult unless your phone is rooted. I am pretty certain that last time I tried, Android wouldn't let me look at the BOINC data files.

What folder would I be looking for?

On the phone's memory /data/data/edu.berkeley.boinc, where the second /data/ is a hidden directory. Even when the phone is rooted, I don't think you can easily write to this directory.

Also "USB Debugging" mode, which is available without rooting, might allow you to see all the files using a PC through a USB cable.

No, since the /data/ directory is a hidden directory, so the PC doesn't see it. Besides, even if you would be able to see it, you only have read rights, no write rights. If it's pre-9 Android, you can try this trick to backup the data directory. I'm sure there's an adb command to push a file back into the hidden directory as well, but you do require write rights for that.

Total Commander will be able to see the files. On all my devices the directory has always been ./data/data/edu.berkeley.boinc, where ./ depicts root.

You started with asking Is there an app_config.xml on android? to which Dave said he wouldn't know why not... but there isn't ever an app_config.xml file unless you make one. And since you can't make one due to lack of write permissions, BOINC won't have one.

Thanks for the very useful info. As you mentioned Einstein on Android is RAM & "disk" hungry.

Android is an absolute shambles.

It is to do with Android being a Linux based and therefore rather more secure setup than Windows. If doing what you want was easy, it would make for a lot of business for the people that fix phones when users have screwed them up. I would like rooting my phones to be less hassle than it is but I can understand the reluctance of those behind the platform to do so.