Generate navigatorEventID by screen

navigatorEventID was generated from navigator, probably by mistake.
This caused a bug which was noticeable when showing modals from
a pushed screen. In this case two components shared the same
navigatorEventID thus they both responded to events from native.

src/platformSpecific.android.js

 function addNavigatorParams(screen, navigator = null, idx = '') {
   screen.navigatorID = navigator ? navigator.navigatorID : utils.getRandomId() + '_nav' + idx;
   screen.screenInstanceID = utils.getRandomId();
-  screen.navigatorEventID = navigator ? navigator.navigatorEventID : screen.screenInstanceID + '_events';
+  screen.navigatorEventID = screen.screenInstanceID + '_events';
 }
 
 function addNavigatorButtons(screen) {