Support r8 Android linker

Summary

In a Xamarin.Forms solution, if I use "Sdk assemblies only" linker option on Android project and Release mode, MediaPlayerElement plays only the audio of the video. Only tested in Android

Minimal project and steps to reproduce

https://gitlab.com/viniciuslln/testvlc/

1. Create a Xamarin.Forms solution
2. Use MediaPlayerElement control
3. Set up "Sdk assemblies only" linker option in Android project and run in Release mode
4. Play some video with http source

Relevant logs and/or screenshots

2019-11-06 21:08:42.268 12468-17380/? E/VLC: [0000007e11a5a390/43e4] libvlc window: AndroidNativeWindow jni init failed
2019-11-06 21:08:42.268 12468-17380/? E/VLC: [0000007e11a5a390/43e4] libvlc window: InitJNIFields failed
2019-11-06 21:08:42.270 12468-17432/? E/VLC: [0000007e0f7fb690/4418] libvlc vout display: parent window not available
2019-11-06 21:08:42.270 12468-17380/? E/VLC: [0000007e17a1ba90/43e4] libvlc video output: video output creation failed
2019-11-06 21:08:42.271 12468-17380/? E/VLC: [0000007e380f1a90/43e4] libvlc decoder: failed to create video output

Environment

• OS: Android
• Version 9
• Device: Xiaomi Redmi 4 Prime / Xiaomi Redmi K20
• LibVLC version and architecture: latest stable
• LibVLCSharp version: latest stable