Create stats_buffer_writer

Move iovec* construction and noteLogLoss from stats_event_list to
stats_buffer_writer.h.
Expose write_buffer_to_statsd that takes in a byte array and writes
it to the statsd socket. Currently exposed for StatsLog.write JNI call.

This change also allows getting rid of stats_event_list once all clients
have migrated to stats_event

Bug: 145619049
Test: m -j libstatssocket
Change-Id: I73f914985421fc1a4f094982c826bf364604b788
Merged-In: I0048e392c2f5039eb70dacf4e91a43d3f32e8749
diff --git a/libstats/socket/Android.bp b/libstats/socket/Android.bp
index b7c07b6..beb009c 100644
--- a/libstats/socket/Android.bp
+++ b/libstats/socket/Android.bp
@@ -20,7 +20,10 @@
 cc_library {
     name: "libstatssocket",
     srcs: [
+        "stats_buffer_writer.c",
         "stats_event.c",
+        // TODO(b/145573568): Remove stats_event_list once stats_event
+        // migration is complete.
         "stats_event_list.c",
         "statsd_writer.c",
     ],