Create an Android keystore file

Add a new Android keystore file to an app. keystore_file_name is required if there is already an existing keystore file for the app. It will determine the environment variable key to be used to refer to the keystore file in builds. E.g. BITRISE_ANDROID_KEYSTORE_<keystore_file_name>_URL. The keystore_file_name can only contain letters, numbers, and underscores.