Fixing default encryption again.

Apple's `[UIDevice identifierForVendor]` proved to be too unstable to use as a long-term static value.  So OAuth and SmartStore will now make use of a unique app identifier that we create, which should be ultra-stable, changing only between app installs.
Merge pull request #401 from khawkins/enc_with_base_app_id

Fixing default encryption again