Isn't 'decided' ,here, functioning as an intransitive verb (ie without a object)?
So the question of whether the verb is active or passive is meaningless.
Using 'it' as the object and 'I' or 'me' as the subject,
the active form of the present perfect verb 'decided' would be:
I have decided upon it.
The passive form would be:
It has been decided upon by me.
:-)