[build] Move global.json file to root directory (#106)

Context: dotnet/java-interop#796
Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4454535&view=logs&jobId=f31c9f97-4411-58e7-49ac-fc73f645e6b6&j=f31c9f97-4411-58e7-49ac-fc73f645e6b6&t=35e41a6f-4381-52eb-e0f2-d95c19469a7f

We're seeing a Java.Interop build attempt fail on
Windows + .NET Framework Windows during a NuGet restore:

	##[error]The nuget command failed with exit code(1) and error(D:\a\1\s\external\xamarin-android-tools\src\Microsoft.Android.Build.BaseTasks\Microsoft.Android.Build.BaseTasks.csproj(4,3): error : C:\hostedtoolcache\windows\dotnet\sdk\3.1.300\Sdks\Microsoft.DotNet.Arcade.Sdk\Sdk not found. Check that a recent enough .NET SDK is installed and/or increase the version specified in global.json.

The "out of repo" restore attempt is failing to find the `global.json`
file next to `Microsoft.Android.Build.BaseTasks.csproj`.

Fix this by moving `src/Microsoft.Android.Build.BaseTasks/global.json`
to `global.json`, next to `Xamarin.Android.Tools.sln`.

Author: pjcollins