Everything related to file system access has been rewritten to make use
of the storage access framework. This means that the
WRITE_EXTERNAL_STORAGE is no longer needed. Because of that, the
dependency on permission_handler could be dropped and all code related
to permission handling has been removed. To be able to open a whole
document tree, the minSdkVersion was bumped to 21. Finally the file
selector was rewritten using custom platform dependent code.
This was done in comparison to a newly created flutter project. Notably
this included migrating to AndroidX, updating Gradle and Kotlin and
introducing a new .gitignore file in the android subdirectory.