ot::OwnedPtr::operator=() method

Overload the assignment operator `=` to replace the object owned by the `OwnedPtr` with another one using move semantics. The `OwnedPtr` first frees its current owned object (if there is any and it is different from @p aOther) before taking over the ownership of the object from @p aOther. This method correctly handles a self assignment (i.e., assigning the pointer to itself).