Propagate an is_zygote flag through nativePostForkChild.

This flag is used to skip calling into Runtime::InitNonZygoteOrPostFork()
in child-zygote processes. Doing so would spin up threads for Binder,
which would make the process unable to fork like a zygote.

Bug: 63749735
Test: m (with multi-project commits landed)
Change-Id: I352af061e0ce6bd32e6f30b15ff2f8d3f4b58222