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: I0048e392c2f5039eb70dacf4e91a43d3f32e8749