Fix #255

https://github.com/dotnet/sdk/issues/3770

It's already in global.json

Builds started failing with the current variable input '3.0.x'

Failed while getting version 3.0.x from channel 3.0
sdk version matching: 3.0.x could not be found

This was partly fixed in https://github.com/videolan/libvlcsharp/pull/88 but not entirely, according to #283. This commit should fix it for good.

Unnecessary and breaks builds with older compilers (C#7 and under)

- close the foreground window
- dispose the PlayerHost
- call dispose in the samples

This patch should be located in the InitializeAndroid method, but it doesn't work if it is there. Must look at generated code
#280

Fixes #275
by storing internal interop callbacks properly.

The DialogCallbacks struct does not need to change either,
so I passed in as static readonly.

TODO
/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/iOS/Xamarin.iOS.Common.targets(646,3): error : No valid tvOS code signing keys found in keychain. You need to request a codesigning certificate from https://developer.apple.com. [/Users/runner/runners/2.163.1/work/1/s/Samples/LibVLCSharp.tvOS.Sample/LibVLCSharp.tvOS.Sample.csproj]

It's built with cake now