Bug 1895781 - Move the App Services and Glean auto-publish logic to its own settings file. r=nalexander

This commit factors out the logic into its own settings file, adds
docs, and only imports the settings file into applications that
consume App Services and Glean via Maven (i.e., not GeckoView).

Differential Revision: https://phabricator.services.mozilla.com/D209897