Remove "task" pluralisation and smurf method naming

[dturner]

Here's what I've done and why:

• Changed the use of "Tasks" in Room and class names from plural to singular. This is consistent with our guidance in training videos (example). Notable changes are:
  • Tasks table -> task
  • TasksDao -> TaskDao
  • TasksRepository -> TaskRepository
  • Changed DAO insert to upsert as it removes the update method
  • Removed "Task" from DAO methods, because smurf naming is an anti-pattern
  • Refactored FakeTaskDao to provide a more robust API for getting/setting the underlying tasks in tests