Zero C++ heap by default.

Let the flakes, and the crashes, and the deadlocks, and the leaks, and
the undefined behavior which steals away so many nights find their
numbers decimated in the homogeneous conditions we now rest our data.

To give some intuition why this is not so underperformant, zeroing
memory is one way of priming caches.

This change is actually a no-op, because build/make/core/soong_config.mk
always overrides this setting, but it reflects the spirit and future
direction of this change.

Bug: 131355925
Test: basic simple perf comparisons
Change-Id: I8254c36373de52091839561973c9741e8f85aa09