Refactor tasks (tasks.org/OpenTasks) to separate layers for storage + mapping

[rfc2822]

As described in https://github.com/bitfireAT/davx5/issues/670

We can use the new event architecture as a template.

• Like LocalCalendar/LocalEvent: provide all fields for DAVx5 #15

  • DmfsTaskList/DmfsTask: provide all fields for DAVx5 #138
  • LocalTaskList/LocalTask: Consume fields provided by synctools davx5-ose#1811

• Like Remove possibility to subclass AndroidEvent #20

  • Remove possibility to subclass DmfsTask #155
  • LocalTask: Don't subclass DmfsTask davx5-ose#1862

• Like Don't allow subclassing of AndroidCalendar anymore #23

  • Disallow subclassing of DmfsTaskList #160
  • LocalTaskList: Stop subclassing DmfsTaskList davx5-ose#1882

• Like Move logic from AndroidCalendar companion object to AndroidCalendarProvider #28

  • Move logic from DmfsTaskList companion object to DmfsTaskListProvider #170
  • DmfsTaskList refactoring davx5-ose#1934

• Like Introduce mapping classes #32

• ...

• ...

• Like Event: extract EventReader/EventWriter #41

• ...

• ...

• Like refactor AndroidEvent; introduce AndroidEvent2 with Entity as data object #40

• ...

• ...

#45

#65

#76

#67

#71

#79

#81

#85

#89

#95

#106

#107