Fix install apk under android 7

lib/index.js

@@ -3,7 +3,12 @@ import {
   getCheckUrl,
   setCustomEndpoints,
 } from './endpoint';
-import { NativeEventEmitter, NativeModules, Platform } from 'react-native';
+import {
+  NativeEventEmitter,
+  NativeModules,
+  Platform,
+  PermissionsAndroid,
+} from 'react-native';
 export { setCustomEndpoints };
 const {
   version: v,
@@ -197,6 +202,18 @@ export function markSuccess() {
 
 export async function downloadAndInstallApk({ url, onDownloadProgress }) {
   logger('downloadAndInstallApk');
+  if (Platform.OS === 'android' && Platform.Version <= 23) {
+    try {
+      const granted = await PermissionsAndroid.request(
+        PermissionsAndroid.PERMISSIONS.WRITE_EXTERNAL_STORAGE,
+      );
+      if (granted !== PermissionsAndroid.RESULTS.GRANTED) {
+        return;
+      }
+    } catch (err) {
+      console.warn(err);
+    }
+  }
   let hash = Date.now().toString();
   let progressHandler;
   if (onDownloadProgress) {