更改安卓端的文件访问方式，优化导入逻辑 (#64)

* Refactor import function & Allow import local comics without copying them to local path.

* android: use file_picker instead, support directory access for android 10

* Improve import logic

* Fix sql query.

* Add ability to remove invalid favorite items.

* Perform sort before choosing cover

* Revert changes of "use file_picker instead".

* Try catch on "check update"

* Added module 'flutter_saf'

* gitignore

* remove unsupported arch in build.gradle

* Use flutter_saf to handle android's directory and files, improve import logic.

* revert changes of 'requestLegacyExternalStorage'

* fix cbz import

* openDirectoryPlatform

* Remove double check on source folder

* use openFilePlatform

* remove unused import

* improve local comic's path handling

* bump version

* fix pubspec format

* return null when comic folder is empty

lib/pages/reader/images.dart

@@ -604,7 +604,7 @@ ImageProvider _createImageProvider(int page, BuildContext context) {
   var reader = context.reader;
   var imageKey = reader.images![page - 1];
   if (imageKey.startsWith('file://')) {
-    return FileImage(File(imageKey.replaceFirst("file://", '')));
+    return FileImage(openFilePlatform(imageKey.replaceFirst("file://", '')));
   } else {
     return ReaderImageProvider(
       imageKey,