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