save/restore instance state of FilesFragment

[wischweh]

FilesFragment does not keep its (navigational) state:

Prerequisite:

• Enable "Don't kill activities"

1. Navigate into a folder.
2. Use Android Task switching to leave the app.
3. return to the app
   Expected:
4. App is in the same navigation level
   Actual:
5. App is back on top level.

States of Dialogs and/or menus (i.e. BottomSheet) is also lost. But this may be considered acceptable.

[audax]

This is only reasonable if we cache the box volumes, otherwise we always need to download multiple DMs before we can show a folder. This is unreasonable.

Depends on Qabel/qabel-core#627