added support for onChange report

Report is now notified when metric changes and updates reading values.

Tested:
  - Added new unit tests
  - OnChange report updates Readings when metric values changes

Change-Id: I3be9ef7aa0486cb15bac627aa1de5cc632613b3b
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/src/sensor.cpp b/src/sensor.cpp
index 2551562..387d0d5 100644
--- a/src/sensor.cpp
+++ b/src/sensor.cpp
@@ -107,17 +107,7 @@
 {
     timestamp = Clock().steadyTimestamp();
 
-    if (value == newValue)
-    {
-        for (const auto& weakListener : listeners)
-        {
-            if (auto listener = weakListener.lock())
-            {
-                listener->sensorUpdated(*this, timestamp);
-            }
-        }
-    }
-    else
+    if (value != newValue)
     {
         value = newValue;