allow system server to control zygote spawned processes

System server needs to be able to tell Zygote to create processes
with differing ids, capabilities, and SELinux security information.
Allow it.

These rules are not in unconfined.te, and as a result, are not
automatically allowed by SELinux in enforcing mode.

Change-Id: I010eaa2b0e0cee5d995e08e6c785cc5e01b2c974