Show focal length when zooming

[vodemn]

Describe the feature or the problem it solves
Having a zoom value in form of "2x" is hard to connect with the lens user has in a real world.

Describe the solution you'd like
Allow users to choose between units for displaying zoom.

Additional context
On Android there is no reliable API to get camera specs without initializing it, so the feature will be disabled until the first measurement is done.