Save/restore seed phrase from Google Credential Manager

[ccjernigan]

This depends on #305

Is your feature request related to a problem? Please describe.

Although users are instructed to backup their seed phrase, it would also be helpful to give users the option to save their seed phrase in their Google account with Smart Lock. This is somewhat similar to the iOS Keychain, although a bit less universal since it depends on Google Play Services.

Describe the solution you'd like

• During onboarding, prompt the user to restore from Google Credential Manager if we find a secret stored there. There is an open question as to whether we prompt at the beginning of onboarding, or wait until the restore/new wallet screen.
• Add the end of the new wallet flow (e.g. after the user restores a seed phrase manually), provide the user the option to opt-in to storing their password with Google Credential Manager.
• Ad the end of the restore wallet flow (after the user has typed in their phrase manually), provide the user the option to opt-in to storing their password with Google Credential Manager.
• Update the Nuke Wallet button to delete the secret from Google Credential Manager, as well as add verbiage telling users that the nuke option will do this.

Alternatives you've considered

Additional context

There may be a few other consent-based options to present to the user, so we should probably consider the design of them together so that we can ask all of our consent questions at the same time. Those other consent-based options might include crash analytics and app analytics.

[HonzaR]

We need to ensure that the system sign-in popup for Google Credentials Manager does not require privacy-sensitive user data.