generate signed apk
So you were wrong. You have to create 1 more key on Google Console for "release version" (singed apk) 'cos it is different than "debug version" which you install on emulator.
This answer on what you provide in your question will solve your problem: https://stackoverflow.com/a/30559898/4316327
First, switch to Project view by using the dropdown in the upper left
of the Project Explorer. Then, expand app/src/, and you will see
subfolders debug and release. Under there, you should see two separate
google_maps_api.xml files under debug/res/values and
release/res/values.
Paste your "release version" key to google_maps_api.xml
inside release
folder.
If that folder is not exist, create one like debug
folder.