Feature request: Register as custom document-provider on android

Since, on desktop keybase can register/mount a custom virtual path/drive. Example `K:\` drive on windows. It would be _nice to have_ the same on android app.

Here's a helpful guide to do that https://developer.android.com/guide/topics/providers/document-provider.html

This would make things very convenient such as saving files directly to public/private folders from 3rd party apps. Or to load files directly.


No repository files, tests, or entry points are named. Start by reading the linked Android Document Provider guide and locating the Android app's storage integration. Done means third-party apps can save files directly to Keybase public/private folders and load files from them through Android's document picker.

Written by the indexing model from the issue text.