metrics: Fix import paths for headers.

This CL fixes the import path to match the Android format.
The libmetrics headers were also moved to include/metrics to make
exporting headers cleaner.

BUG: 22879597

Change-Id: I67d1ac1746cc3560ae4cbddecf97fa48c9dd1480
diff --git a/metrics/metrics_daemon.h b/metrics/metrics_daemon.h
index b1b2d11..04c40e5 100644
--- a/metrics/metrics_daemon.h
+++ b/metrics/metrics_daemon.h
@@ -18,7 +18,7 @@
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
 #include "metrics/metrics_library.h"
-#include "metrics/persistent_integer.h"
+#include "persistent_integer.h"
 #include "uploader/upload_service.h"
 
 using chromeos_metrics::PersistentInteger;