Move Universe::update_heap_info_at_gc to CollectedHeap

In the review for JDK-8248401 the suggestion came up to move Universe::update_heap_at_gc() and related members to CollectedHeap.

void Universe::update_heap_info_at_gc() {
  _heap_capacity_at_last_gc = heap()->capacity();
  _heap_used_at_last_gc = heap()->used();
}

The method also accesses all heap related variables, so CollectedHeap may be a better place to store them.