Use Storage Access Framework on Android

Considering that Keybase keeps encrypted files online and a local secure copy on a device, it would probably make sense to utilize [Storage Access Framework](https://developer.android.com/guide/topics/providers/document-provider) on Android and register as a [document provider](https://developer.android.com/guide/topics/providers/create-document-provider).
This way, the Keybase client would be able to offer files to applications that can utilize the SAF.
Many cloud clients nowadays implement this, which is very convenient. Otherwise, there is no way to really modify the files in KBFS apart from uploading them again.

Start by reading the linked Android Storage Access Framework and document-provider documentation, then inspect the Android client entry points for file access. Done means the client registers as a document provider and lets compatible applications access and modify files in KBFS.

Written by the indexing model from the issue text.