Fix useAppState to workaround unknown state

@see facebook/react-native#18836

Author: tonyxiao

lib/useAppState.js

@@ -1,9 +1,9 @@
 import React, { useEffect, useState } from 'react'
 import { AppState } from 'react-native'
 
-const currentState = AppState.currentState
-
+// https://github.com/facebook/react-native/issues/18836
 export default () => {
+  const currentState = AppState.currentState
   const [appState, setAppState] = useState(currentState)
 
   function onChange (newState) {
@@ -19,4 +19,4 @@ export default () => {
   })
 
   return appState
-}
+}