Refactor and optimize GC code.

Fixed the reference cache mod union table, and re-enabled it by
default. Added a boolean flag to count how many null objects,
immune, fast path, slow path objects we marked.

Slight speedup in mark stack processing, large speedup in image mod
union table scanning.
EvaluateAndApplyChanges Before:
Process mark stack time for full GC only:
12.464089s, 12.357870s, 12.538028s
Time spent marking mod image union table ~240ms.
After:
Process mark stack time: 12.299375s, 12.217142s, 12.187076s
Time spent marking mod image union table ~40ms.

TODO: Refactor reference visiting logic into mirror::Object.

Change-Id: I91889ded9d3f2bf127bc0051c1b1ff77e792e94f