On opt-out, empty the CLIENT_ID, session, and log files

[eliasyishak]

Once the PDD revision has landed, we will need to have the contents of the CLIENT_ID, dart-flutter-telemetry-session.json, and dart-flutter-telemetry.log files deleted when a user decides to opt out. If the user has decided to opt back in, then a new client id will be populated in the CLIENT_ID file, session data will begin to get persisted, and logs will continue to be saved in the log file

• PR to remove the PDD flag: pddFlag removal + tests for pdd restricted instance of Analytics #86
• PR to clear necessary files on opt-out: Clear contents of persisted files on opt out #87

[eliasyishak]

PDD revision was finalized on May 2, PRs are good to get merged