[ReactNative] Fix navigator touch-blocking logic

Summary:
The logic for this is incorrect when the `state.transitionToIndex === 0`, and will return false and not capture the touch.

@public

Test Plan: Try to repro bugs on device and simulator

Libraries/CustomComponents/Navigator/Navigator.js

@@ -1290,7 +1290,7 @@ var Navigator = React.createClass({
         key={this.state.idStack[i]}
         ref={'scene_' + i}
         onStartShouldSetResponderCapture={() => {
-          return !!this.state.transitionFromIndex || !!this.state.activeGesture;
+          return (this.state.transitionFromIndex != null) || (this.state.transitionFromIndex != null);
         }}
         style={[styles.baseScene, this.props.sceneStyle, disabledSceneStyle]}>
         {React.cloneElement(child, {