Move metrics from namespace to class

Add a new metrics reporter interface to handle the current metrics
report for omaha (with metricsd). This helps to add metrics logging
for android phone devices (using other libraries) in the future.

Also change the unit tests accordingly.

Test: unit tests pass for pico
Change-Id: I703c8b6353e4b38bc89d6a71f01623978cf7a549
diff --git a/payload_state.h b/payload_state.h
index 699fc74..98f21e8 100644
--- a/payload_state.h
+++ b/payload_state.h
@@ -24,7 +24,7 @@
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
 #include "update_engine/common/prefs_interface.h"
-#include "update_engine/metrics.h"
+#include "update_engine/metrics_constants.h"
 #include "update_engine/payload_state_interface.h"
 
 namespace chromeos_update_engine {