RESOLVED A day or so after I wrote the original email, I reset the phone to the factory defaults, but Malwarebytes was still alerting me to the same malware. Then after a Malwarebytes update on June 19, the scan was clear and has remained clear. I was not able to confirm this malware with Sophos (see first message) and did not install/try others. ADVICE I got no replies to my posting, which surprised me. BUT many of those posts that do get replies have used tags. So add tags - the more, the merrier. Good luck!
I have been running Malwarebytes Mobile for a while (months) with no problems or issues. However, my routine scan ("Scan after update") came up with a problem on June 13. It identified the malware as Android/Trojan.Spy.Agent.ADI in the file /system/priv-app/Email_master/Email_master.APK, which is a program that can't be deleted. My Malwarebytes log showed a scan the previous evening that completed with no problems. Three app updates were installed automatically through GooglePlay the day I got the malware alert: Calendar, YouTube, and Snapchat. Steps taken: I suspected maybe a