Initial release candidate of the new Android client 🎉

New features/improvements

Completely rewritten app, web client is integrated through a standard Android WebView and custom JavaScript interfaces instead of Cordova
Uses web client as hosted by the server, thus is always up-to-date
Native video player integration based on ExoPlayer, which supports more media formats and codecs, and uses (hardware-accelerated) device decoders
New and improved media notification handling on music playback, should behave much more consistent now

Casting to Google Cast doesn't work, this is an upstream issue in the Jellyfin server, tracked at jellyfin/jellyfin#3852