Serial: Refactor SerialHeap::update_gc_stats and TenuredGeneration::update_gc_stats

During reviewing JDK-8331557, I found that the methods `SerialHeap::update_gc_stats` and `TenuredGeneration::update_gc_stats` only sample the promoted size. So it is good to rename them to `sample_promoted_amount` and simplify `TenuredGeneration::update_gc_stats` (remove the parameter `Generation* current_generation` and reduce the conditional statement).