Includes parallel improvements to Bluetooth key exchange UI.

This commit renames identicons to avatars in field names, and uses an ImageView
derivative to show the identicons, which should make implementing #214 easier.

Also removed unnecessary adapter notifications in various places.

This follows the principle of least surprise: the default identity
doesn't change when a new identity is created.

Returning null from getItem() was causing too many problems, and the
item was triggering onItemSelected() before the other items had been
loaded.

(Although it isn't possible to create an identity yet...)