Fix new_user KPI for two scenarios:

* FirefoxOS was not reporting new users
* If a user hits cancel from the "set password" screen.

fixes #3359
fixes #3459
count new_users who cancel out in the set_password screen
Merge pull request #3460 from mozilla/issue_3359_new_account_kpi

Fix new_user KPI for two scenarios

@kparlante gives the r+, I merge. Thanks @kparlante!