Clear contents of persisted files on opt out

[eliasyishak]

When the user opts out of telemetry collection, we will clear the contents of the following files:

• CLIENT_ID
• dart-flutter-telemetry.log
• dart-flutter-session.json

If the user chooses to opt back into telemetry collection, these files will be regenerated:

• CLIENT_ID – a new client id will be regenerated
• dart-flutter-telemetry.log – events will begin to get persisted locally again as they are sent
• dart-flutter-session.json – the json object containing the session_id and last_ping will be regenerated

Reference issue: #84

[eliasyishak]

@jcollins-g this is the second PR that is required by the PDD to clear the files on opt out – I have landed the previous one that removed the pddFlag already

[bwilkerson]

Assuming that there's a good reason for leaving empty files on the disk, lgtm.

[bwilkerson]

I would have assumed that the files would be deleted from disk, not just emptied. Is there a reason for leaving an empty file on disk?

[eliasyishak]

We asked to have it be cleared because I didn't want to change the initializer logic, on startup, the Initializer class checks to make sure each of these files exist, if not, it will create them. And this happens before we determine if the user has opted out so it would be a lot more refactoring