Implement the update API for GoogleCalendarManager

Why is this change necessary?
- In order to fulfil CRUD requirements, Database and Logic are working on
  implementing an Edit function into taskbuddy. The activities carried out
  by the Edit function need to be synced to Google Calendar.

How does it address the issue?
- A public API offered by GoogleCalendarManager, update(Task), is now
  available for database to call. This update method is used to
  synchronise the edits made to tasks in taskbuddy, with Google Calendar.