Android: browse documents anywhere in the sdcard

Change-Id: I9e52489e13ed0eaeb5926347168de926cd9178b0
Reviewed-on: https://gerrit.libreoffice.org/13277
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>