Contacts: Fix broken export-to-storage function

The contacts exporter relied on a device configuration option that
was hardwired to the older storage path format. Instead of updating
the path, replace its usage with Environment.getExternalStorageDirectory,
which is consistent with how the importer does it.

Change-Id: Icbd06072ddfc3f30c7caffbbfccbcf9df0f0d9b3