Account malloc mmaped pages as native heap.

Currently when malloc uses a page rather than brk the heap gets accounted as
ashmem memory. Previously the memory would be accounted as unknown but since
[1] it gets accounted as ashmem. This change makes the accounting see this
memory as being native, ie the same kind of memory we associate with brk.

[1] https://android-review.googlesource.com/#/c/49583/3

Change-Id: I6f496cfeb590268d41d8e047b12cd5c90359e2b0