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 test
Bug 982663: Part 2 - Integration tests for App Usage metrics. r=etienne
an unrelated fix needed to get the bootstrap test to pass