According to Experience Manager Mobile Help / Sign iOS and Android Apps:
"For Android apps, if you specify a .p12 certificate, specify the password."
I have a p12, and I've specified the password. The Private Key Alias field and Private Key Password field are disabled, and yet, I receive the error:
"Invalid Private Key Alias"
I was using a p12 file that I had generated a couple of years ago and had been signing apps with. I followed the instructions at Android Publishing Guide, and was able to sign the app successfully.