Add zygote init script to mainline_system.mk

We may add all possible scripts to "root" to clean up the whitelists
of mainline devices.
One of the zygote init scripts is selected by "ro.zygote" property.

Bug: 143121605
Test: build test for all device targets
Change-Id: I4435f1c48a306c3a8d42522ed780d405a32780b4