Split the zygote logic out of android.os.Process into a new ZygoteProcess class.

There is no functional change. This is to support adding new types of zygotes
that all operate using the same protocol.

Bug: 21643067
(cherry picked from commit 94e824bc)

Merged-In: Ie673ee816cae34ac20ffb8c774ec3e6461c3fd0a
Change-Id: I104e6133a90dc93a9854836b5e92d3cd542163a3