Add otPlatRadioGetReceiveSensitivity() API (#1715)

* Add otPlatRadioGetReceiveSensitivity() API

* Add a new otPlatRadioGetReceiveSensitivity() API to get sensitivity value;

* Use the receive sensitivity value as the noise floor for link metric computation;

* Remove some unused noise floor related functions.

* Add SPINEL_PROP_PHY_RX_SENSITIVITY Spinel/NCP property

tests/unit/test_platform.cpp

@@ -325,6 +325,11 @@ extern "C" {
         (void)aPower;
     }
 
+    int8_t otPlatRadioGetReceiveSensitivity(otInstance *aInstance)
+    {
+        (void)aInstance;
+        return 0;
+    }
     //
     // Random
     //