Return total footprint, not high water mark.

The mallinfo usmblks value returned by dlmalloc is a little misleading.
It's not the current max, it's the historical high water mark. This
leads to dumpsys meminfo producing native memory numbers that don't add up.
Change this to the real total footprint, not this high water mark.

Bug: 17265653
Change-Id: Id0293a1b50c9b0be8795405049f537a51ab0e8b7