Set up StatsCompanionService.java

Introduces StatsCompanionService.java and sets it up as a system service.

This service is a helper for statsd. It will be responsible for setting
and cancelling alarms related to polling stats and anomaly detection.

It currently does not have selinux permission, so must be started
manually (or with selinux temporarily disabled).

Test: disable enforcing selinux, then set and cancel alarms and make
sure they work.

Change-Id: I3bd73acdd998ee424696cce40965134c14220d8f