Expose AStatsSocket_close() from the statsd apex

As part of statsd becoming a Mainline module in R, we need to formalize
the statsd APIs called from outside the statsd apex.

Bug: 145923416
Test: m
Change-Id: I296a018cc93d95bda77d95f8161acbbe983a5d51
diff --git a/libstats/socket/Android.bp b/libstats/socket/Android.bp
index f36b214..a709961 100644
--- a/libstats/socket/Android.bp
+++ b/libstats/socket/Android.bp
@@ -25,6 +25,7 @@
         // TODO(b/145573568): Remove stats_event_list once stats_event
         // migration is complete.
         "stats_event_list.c",
+        "stats_socket.c",
         "statsd_writer.c",
     ],
     host_supported: true,