Added default save location preference.

Fixes #6

Default file save location of imported dive xml and logs. This path is appended
to default external storage.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
Close UsbDevice on finish

After finishing import, finish the USB device.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>