Only allow system_server to send commands to zygote.

Add neverallow rules to ensure that zygote commands are only taken from
system_server.

Also remove the zygote policy class which was removed as an object manager in
commit: ccb3424639821b5ef85264bc5836451590e8ade7

Bug: 19624279

Change-Id: I1c925d7facf19b3953b5deb85d992415344c4c9f