fix dns create file error

Author: YangSen-qn

library/src/main/java/com/qiniu/android/collect/Config.java

@@ -78,7 +78,7 @@ public final class Config {
     /**
      * dns缓存信息目录
      */
-    public static String dnscacheDir = Environment.getExternalStorageDirectory() + File.separator + "dnschache" + File.separator;
+    public static String dnscacheDir = ContextGetter.applicationContext().getCacheDir().getAbsolutePath() + "/qiniu/dnsCache/";
 
     /**
      * preQuery host

library/src/main/java/com/qiniu/android/storage/persistent/DnsCacheFile.java

@@ -20,7 +20,7 @@ public class DnsCacheFile implements Recorder {
     public DnsCacheFile(String directory) throws IOException {
         this.directory = directory;
         f = new File(directory);
-        if (f.isDirectory() || !f.exists()) {
+        if (!f.isDirectory() || !f.exists()) {
             boolean r = f.mkdirs();
             if (!r) {
                 throw new IOException("mkdir failed");