修复全新安装恢复备份失败bug

* make sure `files/backup` folder created before restore backup

app/src/main/java/io/legado/app/help/storage/Backup.kt

@@ -27,7 +27,9 @@ import java.util.concurrent.TimeUnit
 object Backup {
 
     val backupPath: String by lazy {
-        appCtx.filesDir.getFile("backup").absolutePath
+        val path = appCtx.filesDir.getFile("backup").absolutePath
+        FileUtils.createFolderIfNotExist(path)
+        path
     }
 
     val backupFileNames by lazy {
@@ -182,4 +184,4 @@ object Backup {
             }
         }
     }
-}
+}