completing tasks in aCalendar+ tasks (Android) doesn't properly update BORG for repeating todos

[mikeberger]

aCalendar+ updates the DUE property in VTODOs to move along repeating tasks when they are completed.

BORG is ignoring the DUE property because it prioritizes DTSTART over DUE.

BORG should take DUE as the new todo date over DTSTART when syncing an incoming VTODO.