增加路径不可以写的判断，下载时，如果路径不可写，将执行失败回调 https://github.com/AriaLyy/Aria/issues/750

PublicComponent/src/main/java/com/arialyy/aria/util/FileUtil.java

@@ -691,7 +691,7 @@ public class FileUtil {
     return list;
   }
 
-  private static boolean canWrite(String dirPath) {
+  public static boolean canWrite(String dirPath) {
     File dir = new File(dirPath);
     if (dir.canWrite()) {
       return true;