refactor(form, ngModel): streamline how controls are added to parent forms

This delegates setting the control's parentForm to the parentForm's
$addControl method. This way, the model controller saves one instance
of looking up the parentForm controller. The form controller keeps two
lookups (one for its own ctrl, one for the optional parent).

This also fixes adding the parentForm in the following case:
- a control is removed from a parent, but its corresponding DOM
element is not destroyed
- the control is then re-added to the form

Before the fix, the control's parentForm was only set once during
controller initialization, so the the parentForm would not be set on
the control in that specific case.