Use dependabot multi-directory configuration for Android example gradle updates (#8036)

[Multi-directory config](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#configuration-options-for-the-dependabotyml-file) allows duplicated dependabot configs can be removed in favor of one that specifies multiple directories.  Update the Android example apps so they can be updated all at once.

Also updated the dependabot CI command to look for either `directory` or `directories` key.

Part of flutter/flutter#148098

Author: jmagman