Add test to verify external account settings is shown.

- add a test service that provides a test authenticator and a test
preference screen. Creates corresponding test account and verifies that
the preferences defined in the authenticator is displayed from the
account detail page.

Change-Id: Iab87177ae500d0a36272cf8ecf464139a3dc07cd
Fixes: 35353708
Test: make SettingsUnitTests