fix: Fix call event listener name + Added Call example (#1886)

* fix: bugfix + example

* relocate global variable

src/util/Constants.js

@@ -51,7 +51,7 @@ exports.Events = {
     DISCONNECTED: 'disconnected',
     STATE_CHANGED: 'change_state',
     BATTERY_CHANGED: 'change_battery',
-    INCOMING_CALL: 'incoming_call',
+    INCOMING_CALL: 'call',
     REMOTE_SESSION_SAVED: 'remote_session_saved'
 };