Further reduce statsd memory usage.

+ Remove the protobuf *Metric object from MetricProducers
   -- This saves ~150 bytes per metric.
+ Remove the StatsdConfig from ConfigManager
   -- This saves us xKB per config.
+ Also remove alerts from fake config to avoid crash (Bug: 70627390)
+ Other misc fixes too.

Test: statsd_test & manual
Change-Id: Ied4eb3fa31c50599817b3a5e1caf5077c487fad2