Enable mod union table

Enable mod union table(bitmap). This change aims to reduce the second pause times by processing dirty image cards and cleaning them by storing their alloc space references in a bitmap. The bitmap is then scanned concurrently(if CMS) before recursive mark.

Reduces second pause time. Second pause time now typically < 5 ms instead of 30-40ms.

Change-Id: I831eeb8af7299d4e86cf2dd9c29d0b7e230cd387