Reflect MALLOC_ZERO_CONTENTS default true.

See corresponding build/soong change. This change sets the android
platform to zero all heap allocations by default. To give some
intuition for why this is no so underperformant, zeroing memory is one
way of priming caches.

The main goal of this is to prevent accidental reliance on allocations
being zero, which is UB in C++. In some situations, allocations are
almost always guaranteed to be 0, and so resulting flakes can be
extremely rare.

Bug: 131355925
Test: allocated memory successfully getting zerod
Change-Id: I8c27fbc8c06420a15d022eb810595599d1e56aa0