[sdk] animate header component when app comes back to the foreground

[Tim Peeters]

When the header is hidden by scrolling it out of the view in an app, and the user switches between apps, when an app comes back to the foreground, the header instantly appears again. This means when going back to a previous app, the app is not in exactly the same state as where the user left it.

Example:
- Open gallery-app
- Scroll down to hide the header (you need to have enough photos in the gallery to scroll down).
- Long left/right edge swipe to hide send the gallery-app to the background
- Re-open the gallery app

Result:
The gallery-app re-opens with the header visible.

This can be confusing for the user.
Two possible solutions:
1. Show the header when re-opening the app, but animate it so the user knows what is happening
2. Don't do anything. Header stays hidden and the user can show it as they normally would.

After discussing with Olga, we decided to go for solution 1. The header will show, but it must animate.

---------- UX comment -------------

As described above:
The header should animate once user navigates back to the app.