Guide view: Fix restore fragment state crash

We've had several puzzling crash reports for a while:
`android.os.BadParcelableException: ClassNotFoundException when unmarshalling: android.support.v4.app.FragmentManagerState`

I finally reproduced it:

1. Enable `Settings` -> `Developer options` -> `Don't keep activities`.
1. Open guide view.
1. Click on a part or tool.
1. Hit back.
1. Swipe to other steps and back.
1. Go to 3.

In my experience it takes 2 tries to get it to crash. Turns out it's a
bug in the support library:
https://code.google.com/p/android/issues/detail?id=37484#c1

The upside is that it's an easy fix. The downside is that we have to
extend the class at `android.v4.app.FixedFragmentStatePagerAdapter` and
use that one in place of the default implementation.