Bug 982663: Part 1 - Collect and report app usage data. r=etienne

* record app installation and uninstallation
* send a unique device id with each report
* report deviceinfo, language and screen data, and make telemetry parameters
  configurable through settings
* Refactor to match system app module structure
* Unit tests and bug fixes
* Change strings in Settings and FTU apps to mention app usage data collection
* Address review comments
* Fix failing bootstrap tests
Bug 982663: Part 2 - Integration tests and follow up for App Usage metrics. r=etienne

* Use ftu.pingURL as the default base URL for app usage metrics
* Added tests for app usage default values
* Use manifestURL instead of origin to track unique apps
Merge pull request #21215 from marshall/bug982663_metricsAppUsage

Bug 982663: Collect and report app usage data