Bug 1433334 - Store the install source of the extensions for telemetry. r=aswan

• Implemented stored "install source of the extensions" for telemetry events.
• Added test assertions related to the expected install and addon installTelemetryInfo.
• Added installTelemetryInfo to addon installed from "drag-and-drop" file urls on Firefox Desktop.

Differential Revision: https://phabricator.services.mozilla.com/D4445