Add android support

[mfkl]

Code from https://github.com/xamarin/Essentials/tree/master/Xamarin.Essentials/Clipboard mostly.

[mfkl]

C:\Program Files\dotnet\sdk\3.1.201\Sdks\Microsoft.NET.Sdk\targets\Microsoft.PackageDependencyResolution.targets(234,5): error NETSDK1047: Assets file 'C:\projects\textcopy\src\TestConsole\obj\project.assets.json' doesn't have a target for '.NETFramework,Version=v4.6.1/win7-x86'. Ensure that restore has run and that you have included 'net461' in the TargetFrameworks for your project. You may also need to include 'win7-x86' in your project's RuntimeIdentifiers. [C:\projects\textcopy\src\TestConsole\TestConsole.csproj]
C:\projects\textcopy\src\AndroidApp\AndroidApp.csproj : error MSB4057: The target "pack" does not exist in the project.
Test run for C:\projects\textcopy\src\Tests\bin\Release\net472\Tests.dll(.NETFramework,Version=v4.7.2)
C:\projects\textcopy\src\AndroidApp\AndroidApp.csproj(119,3): error MSB4019: The imported project "C:\Program Files\dotnet\sdk\3.1.201\Xamarin\Android\Xamarin.Android.CSharp.targets" was not found. Confirm that the expression in the Import declaration "C:\Program Files\dotnet\sdk\3.1.201\\Xamarin\Android\Xamarin.Android.CSharp.targets" is correct, and that the file exists on disk.

[mfkl]

I still get UapApp.csproj : error MSB4057: The target "pack" does not exist in the project. locally (after removing the android sample from the sln), unsure why.

Let me know what you prefer, building the android sample on CI or not. I'm not sure that's necessary.

[SimonCropp]

awsome work. thanks

i will look into the CI