sensor_hub: Correct timestamp to elapsedRealtimeNanos

Change-Id: I5d4452b4bf4a185ed06bc6b7c3de5fdfa8729372
diff --git a/sensor_hub/libsensors/SensorBase.cpp b/sensor_hub/libsensors/SensorBase.cpp
index 8c9cf70..6fcf871 100644
--- a/sensor_hub/libsensors/SensorBase.cpp
+++ b/sensor_hub/libsensors/SensorBase.cpp
@@ -109,7 +109,7 @@
 int64_t SensorBase::getTimestamp() {
     struct timespec t;
     t.tv_sec = t.tv_nsec = 0;
-    clock_gettime(CLOCK_REALTIME, &t);
+    clock_gettime(CLOCK_BOOTTIME, &t);
     return int64_t(t.tv_sec)*NS_PER_SEC + t.tv_nsec;
 }