Automatically saving files to `getDirectoryPath()` directories

[derdilla]

On android the ACTION_OPEN_DOCUMENT_TREE intent started when calling FilePicker.platform.getDirectoryPath() grants more permanent permissions than exposed through the file_picker api. Specifically it allows saving files without asking for permissions again, which is useful in the context of reoccurring exports.

My suggestion is that since in FilePicker.platform.saveFile the initialDirectory parameter isn't implemented on android, it could be used to attempt save a file to a previously picked directory.