Added previously missing audio files

Added previously missing audio files
Updated Android SDK version

New Feature: Quizzes!
Practice, train and test your understanding of the most important phrases.

Also fixed a few subtle bugs, e.g. when changing languages or unfolding insights.

We added new sentences, Emergency number information and reworked the Insights section.

Minor fixes, prepared backend code for upcoming features.

We have fixed the back-button issue and reworked some insights.