Add Zygote.startChildZygote() to fork a new process that itself is a zygote.

This adds a new --start-child-zygote argument that instructs the main
zygote to create a new child process that will also be a zygote. The
system_server generates a random name in the abstract socket namespace
for it and the child-zygote to communicate over, and that is passed as
an argument to the new process.

A child-zygote bypasses the normal post-fork-child of the zygote process
in order to preserve itself as a zygote. This means not starting the
Binder threadpool nor launching into ActivityThread. Instead, a
child-zygote calls into its own main function. The main function runs a
ZygoteServer select loop, listening on the socket name specified by the
system_server when it was forked.

Unlike the system zygotes, a child-zygote can be killed without bringing
down the system. Killing a child-zygote will not terminate its child
processes, which will be reparented to init for reaping when they
eventually exit.

Bug: 63749735
Test: m (with multi-project commits landed)
Change-Id: I3e7ebbdba498f8fec1d84cdf927dc43a92be4b68