Enable biometric unlock (onboarding)

[sandysage]

Acceptance criteria

• Should provide a way to skip this step if user chooses not to enable
• On choosing to enable, user should be prompted to authenticate with biometrics
• Only display this to users that:
  1. Have Fingerprint available on their device
  2. Have this feature enabled on their device (have a Fingerprint set).

Make sure that navigating to this screen prevents the user from seeing the ugly webpage error as referenced in #273

requires #28

Engineering Notes

On iOS, this required "pausing" in driving the UI from the datastore state while onboarding. We added a val onboarding: Observable<Boolean> to the RouteStore, and ignored incoming non-onboarding routes while it was true.

Visual Design

InVision
Initial: https://mozilla.invisionapp.com/share/3SN35UHWJDX#/309979870_Fingerprint
Error: https://mozilla.invisionapp.com/share/3SN35UHWJDX#/310610373_Fingerprint_-_Error
Success*: https://mozilla.invisionapp.com/share/3SN35UHWJDX#/311048434_Fingerprint_-_Success
*Intent would be that the success message displays, pauses for a second, then transitions to next screen in onboarding (AutoFIll)

Zeplin
Initial: https://zpl.io/VYBxqRk
Error: https://zpl.io/Vk5l1nW
Succes: https://zpl.io/25qK4rW