Bug 951304 - Follow-up: Make UI look better on old tablets.

This patch restricts the UI width, and centers the content horizontally
and vertically.
Bug 951304 - Follow-up: Stretch passwords on demand.

This postpones the expensive computation until after the interstitial
spinner is displayed.
Bug 951304 - Follow-up: Look better on tablets; improve interstitial. r=rnewman
Bug 956581 - Make FxAccountGetStartedActivity a legitimate AuthenticatorActivity.
Bug 956581 - Make FxAccountGetStartedActivity an AuthenticatorActivity. r=rnewman
Bug 951264 - Part 1: Add FxAccountCreateAccountNotAllowedActivity.
Bug 951264 - Part 2: Show reasonable year ranges in age dialog.
Bug 951264 - Part 3: Add FxAccountAgeLockoutHelper.
Bug 951264 - Part 4: Lock out creating Firefox Accounts if age check has failed recently.
Bug 951264 - Review comments.