Some fixes in StatsdStats, and add some unit tests

+ Add timestamp for when metric data is reported.

Test: statsd_test

Change-Id: Ief5ec5172feed4ec74b7422b77cf69ec8361ef2f
diff --git a/cmds/statsd/src/StatsService.cpp b/cmds/statsd/src/StatsService.cpp
index 8b64f0d..7a127e0 100644
--- a/cmds/statsd/src/StatsService.cpp
+++ b/cmds/statsd/src/StatsService.cpp
@@ -488,7 +488,7 @@
     if (args.size() > 1) {
         reset = strtol(args[1].string(), NULL, 10);
     }
-    vector<int8_t> output;
+    vector<uint8_t> output;
     statsdStats.dumpStats(&output, reset);
     return NO_ERROR;
 }