It has taken me a couple of hours, but I have Mailvelope, APG and K9mail working on my Android phone as well as on my laptop.    I finally figured out that there is a hidden tab that allows APG to import keys from keyservers.   For K9mail, since I use two-factor authentication with gmail, I had to set up an "application-specific password."   I put my secret key on my phone by MTPFS mounting it, copying the ASCII-armored secret key to Downloads folder, importing it into APG, and then remounting the folder to delete it.  

Useful links:
https://www.minertechsolutions.com/blogs/how-to-configure-your-android-phone-with-gmail-using-k-9-mail-more/

https://android.stackexchange.com/questions/54559/how-do-i-setup-a-gmail-account-with-2-step-verification-in-k-9-mail