Use document provider to grant third-party apps access to app-private directories

With the introduction of File Manager where an elegant method has been developed to support opening any files from any directory via a content provider so that the recipient app does not require any file or root permission, it is possible to enhance this support via a document provider so that a third-party app can access app-specific private directories such as `/data/data/package.name` without root permission. This could greatly reduce the attack surface as fewer number of apps would need to use the root permission.

Possible use cases:

1. Editing XML files at `/data/system/ifw/` without granting root or even the editors that do not support root but support SAF (AKA document provider).
2. Granting file managers access to certain app directory, such as `/data/data/io.github.muntashirakon.AppManager/files/profiles` so that you can add/edit profiles directly.
3. Backing up certain directory of interest via third-party event handling apps such as Tasker without letting them know about the location of the files and/or without granting root
4. I forgot others, but the general idea is to minimise apps that can access root.