Allow zygote to write to statsd and refactor

We plan on migrating MetricsLogger to write to statsd socket. So we need to
allow zygote, which writes to logd using MetricsLogger, to also be able
to statsd. We also re-locate some sepolicies to write to statsd socket
in their respective policy definitions.

Bug: 110537511
Test: no failure/violations observed
Change-Id: I21fd352a25ed946516f9a45ac3b5e9bf97b059bc
diff --git a/private/system_server.te b/private/system_server.te
index 6d3d1e3..e916415 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -167,6 +167,9 @@
 unix_socket_connect(system_server, racoon, racoon)
 unix_socket_connect(system_server, uncrypt, uncrypt)
 
+# Allow system_server to write to statsd.
+unix_socket_send(system_server, statsdw, statsd)
+
 # Communicate over a socket created by surfaceflinger.
 allow system_server surfaceflinger:unix_stream_socket { read write setopt };