To make a android apk in flash builder is having very simple steps.
Export release build
and build the project and you got your apk. Here the certificate are very easy to create, choose create and fill the details and save the path.
When I was working on a flash project I lost my certificate some how. and when I was trying to update my app in Google play store, its throwing an error that different signing certificates. I meshed up with the name of certificates i am having. So I tried with many certificates I have but their are around more them 30 certificates. If you have lost your certificate or delete it and you want to update your app in play store you can't because once you have upload with one certificate it will always ask for the same otherwise it will reject your app.
If your certificate is just misplaced and you are not sure which one will be the original certificate then this blog will help you identify you certificate.
You should have the apk which you have upload first time or download the apk from Google play store(it will be the same) now here is the command to check the signature of certificate used in the app
using the Java keytool:
C:\Users\sj> cd C:\Program Files (x86)\Java\jdk1.8.0_60\bin [ENTER]
Place your both apk to the jdk bin folder so that the above path can access it. Now you get the sign of both the app. compare and check which apk sign are matching with the original app. You just have to create the apk with all certificate you are having and check the sign, I know this is a lengthy process but this is the only and easy way to find your certificate if they are misplaced , If they are lost or delete then there is no way to get it back. You have to create the apk with new name and with new certificate.