Add new build type.

This new build type creates a system image that defaults to 64 bit
only, but all 32 bit code is present. In addition, changing the
properties on the vendor partition will allow the secondary zygote
to start as if it was a 64_32 image.

Bug: 227482437

Test: Verified that secondary doesn't start with same disk image
Test: with ro.zygote set to zygote64 and abilists set appropriately.
Test: Verified that secondary does not start when restarting netd.
Test: with ro.zygote set to zygote64_32 and abilists set appropriately.
Test: Verified that secondary does start when restarting netd.
Change-Id: I8779b548e45bd5b32bd4c8b50010ef61a5f4e99a