Separate pub root preference logic, from lifecycle

The custom pub root directory code in https://github.com/flutter/devtools/blob/master/packages/devtools_app/lib/src/shared/preferences.dart

Is tightly coupled with lifecycle triggers from the servicemanager, and isolate managers.

To make the code cleaner, and more testable, separate the lifecycle logic from the preference logic. This may require moving the custom pub root directory code to a separate class and then triggering the logic in those lifecycle triggers.