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

pubspec.lock

@@ -389,6 +389,15 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.5.1"
+  flutter_saf:
+    dependency: "direct main"
+    description:
+      path: "."
+      ref: HEAD
+      resolved-ref: "51a27e2ca0e05becfb8ee3a506294dc4460721a8"
+      url: "https://github.com/pkuislm/flutter_saf.git"
+    source: git
+    version: "0.0.1"
   flutter_test:
     dependency: "direct dev"
     description: flutter