DmfsTask: move all Task mapping out of storage layer

[rfc2822]

Part of #116. Depends on #340 and #342.

Analogous to: events cleanups synctools #89, synctools #95, synctools #107

After sub-issues #340 and #342, all mapping logic lives in mapping/tasks/. This issue cleans up any remaining coupling.

synctools changes

• Ensure storage/tasks/ has no remaining imports of ical4android.Task (the iCalendar data class)
• Builder/handler classes operate on ContentValues/Entity ↔ VTODO directly, without Task as intermediary where possible
• Apply any task-specific correctness fixes analogous to events Only generate recurrence fields/properties for recurring main events #89 and Ignore recurrence rules whose UNTIL is on or before DTSTART #95, e.g.:
  • Filter invalid RRULE (UNTIL before DTSTART) if applicable
  • Only generate recurrence fields for tasks that actually have recurrence rules
• Remove remaining mapping residue from storage/tasks/DmfsTask.kt

DAVx5 changes

Possibly small interface-level adjustments if method signatures change.