Trampoline biometric auth activity

[ignoramous]

Issues like auto-cancellation of auth are still prevalent and so, impl a new activity just for auth and trampoline the user to it (the way we do for Pause mode), should make auto-cancellation not dismiss the app itself:

Unfortunately Android doesn't let me capture it, but when working correctly, you open Rethink after a while since last authenticating, and you get three "layers":

1. Rethink's Home screen
2. the Android prompt on top of that, asking to provide biometric auth, including a Password button to fall back to that
3. the Android fingerprint reader itself, lit up at the bottom of the screen

What I'm seeing when launching from recents is this:

• Rethink's Home screen opens
• a moment later it closes again, returning me to my Android launcher
• an Authentication cancelled toast from Rethink shows
• the biometric prompt then appears over my launcher, but the fingerprint reader doesn't light up at all

Effectively I end up with only layer 2 from above.

Originally posted by @Terrance in #1058 (comment)