Fixed issue with wrong timestamp

Telemetry service used steady_clock for generating timestamps, but it
produced incorrect time. This change makes telemetry service use
steady_clock for intervals and system_clock for timestamps.

Changed readings timestamp to display current timestamp instead of a
time when reading was received.

Tested:
- correct timestamp is visible on dbus
- other telemetry service features are still working

Change-Id: Ic49f45640532cfffaeff5e0bd5591e6d99e5def5
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/src/sensor.cpp b/src/sensor.cpp
index 5c71910..d81e1a1 100644
--- a/src/sensor.cpp
+++ b/src/sensor.cpp
@@ -100,7 +100,7 @@
 
 void Sensor::updateValue(double newValue)
 {
-    timestamp = Clock().timestamp();
+    timestamp = Clock().steadyTimestamp();
 
     if (value == newValue)
     {