Add support for only starting 64 bit zygote.

Added code to support a single system image that can start the
64 bit zygote only, or both the 64 bit and 32 bit zygote. By
default, the system will default to 64 bit only.

For this image, all abilist properties are only present in the
vendor partition. All other copies of these properties are deleted.

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.
Test: Verified that a 64 bit device only starts the primary.
Test: Verified that a 32 bit device only starts the primary.
Change-Id: Ifc6413cecc4df8ba041699f7d0b10786d1926d64