In the alexa-avs-sample-app ca.crt and api_key.txt file is missing how get or how to create these files
All of this information and more is in the README for the project. Below are links to the parts you are asking about, but you should really follow they whole guide to ensure everything is just right.
For generating the self signed cert on:
For the api_key.txt:
Android App and Amazon Alexa Speaker 1 Answer