investigate locking and unlocking app with face id

[michaelneale]

would like to be able to lock and unlock with some biometric like faceid, but in the flutter idiomatic way (it can delegate to the platform).
if easier, can start with android.
The app should have an option to be locked until a biometric (such as face) is provided.
When it is unlocked, should show the current functionality like it was before.

Would like implementation and unit test for the new feature.