Fix ddms heap monitoring.
Fix ddms heap monitoring.

The bug was that the struct malloc_chunk 'head' field only contains a
chunk length if the bottom bits signify that the chunk is actually in
use. But rather than fix our logic, we can just call mspace_usable_size.

Change-Id: I10648f0486c19555061d0e4772668528dd8f076b

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst