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/timer.cc b/metrics/timer.cc
index 99f68fe..ce4bf67 100644
--- a/metrics/timer.cc
+++ b/metrics/timer.cc
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "metrics/timer.h"
+#include "timer.h"
 
 #include <string>