Split fork and specialize in zygote

Separate the portions of the zygote jni library that perform forking
from specialization into system_server or app process.  This is a small
step towards separating forking into a pre-launch activity, possibly
with a pool of pre-application processes.

Bug: 68253328
Test: boot and run
Change-Id: I6ce694d9a69bf7d10cf61cd39989091ee9cabd37
(cherry picked from commit c5c8bcd5)