altosdroid: Switch storage to media dir

Even with the WRITE_EXTERNAL_STORAGE permission, we appear to no
longer have permission to write to a random external storage
dir. Instead, we only have permission to write to an app-specific dir,
buried deep in the directory tree. We can get that directory name with
getExternalMediaDirs()[0].

Signed-off-by: Keith Packard <keithp@keithp.com>