Add elapsedRealtimeNano(), and use CLOCK_BOOTTIME where possible.

Change-Id: I4231c8ca32663e6e2cb5b7c126d091f837373807
diff --git a/libs/utils/Timers.cpp b/libs/utils/Timers.cpp
index 64b4701..d4f8516 100644
--- a/libs/utils/Timers.cpp
+++ b/libs/utils/Timers.cpp
@@ -39,7 +39,8 @@
             CLOCK_REALTIME,
             CLOCK_MONOTONIC,
             CLOCK_PROCESS_CPUTIME_ID,
-            CLOCK_THREAD_CPUTIME_ID
+            CLOCK_THREAD_CPUTIME_ID,
+            CLOCK_BOOTTIME
     };
     struct timespec t;
     t.tv_sec = t.tv_nsec = 0;