ART: use freed_bytes in reclaimed_bytes_ratio calculation

This commit uses actual bytes reclaimed in the current GC cycle instead
of difference of num_bytes_allocated_ before and after GC cycle for
reclaimed_bytes_ratio calculation. The latter could be affected by
mutators running concurrently to GC cycle.

Test: Run art with -XX:DumpGCPerformanceOnShutdown on some benchmarks.
Bug: 112187497
Change-Id: Icc845566bfe14706b0a4a60ee33b64a4f5e66942