Adjust 'account unlock' scenario to make 'forgot pattern' more forgiving.

When a user hits the 'forgot pattern' button, they aren't permanently forced into
using the account to unlock the screen; they can get back to the pattern screen by
hitting the back key, or by turning off the screen (e.g it's not sticky).

Also, show progress dialog while checking account, and make sure we are on the UI thread
when doing stuff from the account callback.