Add StatsEvent.Builder

Use a builder to construct StatsEvent objects.
Make StatsEvent immutable.
Wrap socket byte array in a Buffer class with write methods for
primitives.
Implement socket schema at go/statsd-socket-doc
StatsEvent.java design doc at go/statsevent-java-doc

Bug: 141696033
Test: m -j
Change-Id: Ief4890882de66d5dcca71b3827ffa1f43ea628a8