[HWASan] Enable zero-initialization.

Previously, we use to fill memory with 0xbe bytes. This caused a lot of
problems that necessitated disablement. For example, 0xbe-filled mutexes
are apparently locked, and there were a few instances of
uninitialized-mutex use.

Given that zero-fill is now the default behaviour, enable zero-init in
HWASan as well.

For now, only fill the first page. It would be preferable to fill the
whole allocation, but I don't want to spin for too many cycles filling
huge secondary pages. In future, we might change the behaviour to have
an explicit "zero initialize" option that completely fills the primarily
allocations, and knows it's unnecessary for the secondary.

Bug: 226078464
Test: Boot w/ HWASan (done by presubmit robot)

Change-Id: I7de3a7f9fa2fdeb5116e5bf6586babe4d06fcb91