Coerce AppState.isAvailable to a boolean

Close #2674

Author: mozzius

packages/react-native-web/src/exports/AppState/index.js

@@ -34,7 +34,7 @@ const AppStates = {
 let changeEmitter = null;
 
 export default class AppState {
-  static isAvailable = canUseDOM && document[VISIBILITY_STATE_PROPERTY];
+  static isAvailable = canUseDOM && !!document[VISIBILITY_STATE_PROPERTY];
 
   static get currentState() {
     if (!AppState.isAvailable) {