To pass the active id of an object use res_id instead.
<field name="context">{'res_id': active_id}</field>
The complete code would be
<a type="action" name="action_id">
<!-- Anything Here -->
</a
<record model="ir.actions.act_window" id="action_id">
<field name="name">demo</field>
<field name="res_model">demo</field>
<field name="view_type">form</field>
<field name="view_id" ref="my_module.demo_view_id"/>
<field name="view_mode">form</field>
<field name="context">{'res_id': active_id}</field>
</record>