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)
(cherry picked from commit 96b49848)
Change-Id: I2e12057e4c2e7567f909d699b487e70b1664cca8