Zygote: Load system server code early

In an effort to allow loading integrity-checked artifacts from
the dalvik-cache, attempt to create and cache the system server's
classloader early, while still being in the system_server_startup
selinux domain.

The advantage of this approach is that allowances for loading
from the cache are restricted to startup.

Bug: 128688902
Test: m
Test: Device boots, picks up /system artifacts
Test: Device boots, picks up integrity-checked /data artifacts
Change-Id: If4a75fa106db09f1bd666d6d8df7ac3ac3e35a8c