aura: Dont have oUF aura specific logic inside CreateIcon

If a user overrides CreateIcon then they must also implement
oUF specific logic to make the rest of the aura code work.
Namely, inserting the new icon into icons and incrementing
createdIcons which are used by oUF's external aura code.

These two operations can be pulled out of oUF's implementation
of createAuraIcon and allow overrides to not need to worry about
internal oUF behaviour.