[SPARK-9747] [SQL] Avoid starving an unsafe operator in aggregation

This is the sister patch to #8011, but for aggregation.

In a nutshell: create the `TungstenAggregationIterator` before computing the parent partition. Internally this creates a `BytesToBytesMap` which acquires a page in the constructor as of this patch. This ensures that the aggregation operator is not starved since we reserve at least 1 page in advance.

rxin yhuai

Author: Andrew Or <andrew@databricks.com>

Closes #8038 from andrewor14/unsafe-starve-memory-agg.