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_mock.h b/metrics/timer_mock.h
index 2f2d0f4..ed76f12 100644
--- a/metrics/timer_mock.h
+++ b/metrics/timer_mock.h
@@ -9,7 +9,7 @@
 
 #include <gmock/gmock.h>
 
-#include "metrics/timer.h"
+#include "timer.h"
 
 namespace chromeos_metrics {