Android: prevent NPE due to lack of permissions on a directory

Change-Id: Ia2f4c04ad4c524e8b72870acb0735793624f0916
Reviewed-on: https://gerrit.libreoffice.org/13278
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>