[Feature request] FUSE/KBFS for Android

KBFS uses FUSE, which is a way for userspace code to implement their own filesystems. Android is a Linux-based OS, so it has the potential to support FUSE. [This has been done before](https://github.com/seth-hg/fuse-android), although that was quite a while ago, but if I'm not mistaken modern Android uses FUSE internally. But in a way that normal apps cannot access it... but the root user might.

File operations like moving files are not supported on Android. If it becomes possible to use FUSE to allow root users to mount KBFS somewhere, and then access it like you access the external SD card, then we wouldn't need to implement file operations in the app.

No files or tests are named. Start by investigating the Android FUSE constraints described in the issue and the linked fuse-android project, then determine whether a root user can mount KBFS and access it like external storage; done means supported file operations through that mount.

Written by the indexing model from the issue text.