Rename FxUI to FxAccountsUI

(cherry picked from commit 30ceb2c0c37aefea8e30c951521b1a78d29b3876)
Ensure that the UI flow always returns a result

(cherry picked from commit af18a145e670a9f5df5db4b2e74ca602fbb52462)
Merge pull request #52 from ferjm/fxa_proposal_complete

s/FxUI/FxAccountsUI :) and return flow result to chrome