Add support for only starting 64 bit zygote.

This is part of the changes that will allow creating a single
system image but a different set of properties will either
start or not start the secondary zygote.

Bug: 227482437

Test: Verified that secondary doesn't start with same system image
Test: with ro.zygote set to zygote64 and abilists set appropriately.
Test: Verified that secondary does not start when restarting netd.
Test: Verified that secondary does start with same system image
Test: with ro.zygote set to zygote64_32 and abilists set appropriately.
Test: Verified that secondary does start when restarting netd.
Test: Verified that a 64 bit device only starts the primary.
Test: Verified that a 32 bit device only starts the primary.
Change-Id: Id37a223c73f9a61868b2e26450ef4b6964f7b496