[query] Fix randomness handling in StreamAgg (hail-is#14838)

`uses_agg_randomness` is a method, not a property, so we need to call it
in the StreamAgg `__init__`, otherwise, we're just passing the bound
method object or using it as a boolean.

## Security Assessment
- This change has no security impact

### Impact Description
Internal API change

Author: chrisvittal

hail/python/hail/ir/ir.py

@@ -2261,8 +2261,9 @@ def renderable_bindings(self, i, default_value=None):
 class StreamAgg(IR):
     @typecheck_method(a=IR, value_name=str, body=IR)
     def __init__(self, a, value_name, body):
-        a = a.handle_randomness(body.uses_agg_randomness)
-        if body.uses_agg_randomness:
+        uses_agg_randomness = body.uses_agg_randomness(is_scan=False)
+        a = a.handle_randomness(uses_agg_randomness)
+        if uses_agg_randomness:
             tup, uid, elt = unpack_uid(a.typ)
             body = AggLet(value_name, elt, body, is_scan=False)
             body = with_split_rng_state(body, uid, is_scan=False)