Viewer for Khan Academy

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Fixed several crashes, including:
- an image-related crash when resuming to foreground on 4.4
- the app could crash when getting an error response from Khan Academy servers
- if logged in, the app would frequently crash a few seconds into video playback

Fixed some bugs:
- subtitles were no longer downloading
- when navigating back to a list view, your scroll position is now restored
- energy points should once again be awarded for watching videos