Add elapsedRealtimeNano(), and use CLOCK_BOOTTIME where possible.

Change-Id: I4231c8ca32663e6e2cb5b7c126d091f837373807
diff --git a/include/utils/SystemClock.h b/include/utils/SystemClock.h
index 7c319be..d75264c 100644
--- a/include/utils/SystemClock.h
+++ b/include/utils/SystemClock.h
@@ -25,6 +25,7 @@
 int setCurrentTimeMillis(int64_t millis);
 int64_t uptimeMillis();
 int64_t elapsedRealtime();
+int64_t elapsedRealtimeNano();
 
 }; // namespace android