Zygote: disable heap 0'ing in apps.

Uninitialized memory back by popular demand.

We have trialed heap 0'ing (malloc is calloc) in Android for a few days
in all processes, but there are a couple of signs that doing this in app
processes may be too ambitious.

Bug: 170891666
Bug: 131355925

Test: boot, see disable logs
Change-Id: I60563712c8af459f2f847b4598477b042a05d7ea