The new `layouts` preference replaces three previous preferences:
layout
second_layout
custom_layout
Add a preference migration function, which first migration is to
migrate layouts into the new preference.
The migration must also be called from the SettingsActivity as it might
use a different preference store due to the boot-aware preference copy.
This new class will help write more logs. The LogPrinter is no longer
created everytime the keyboard is opened.
An error log is added if failing to load the custom extra keys.
