Add Android default global usings in MSBuild targets

[mhutch]

Following the pattern in dotnet/sdk#18459, the Android SDK targets should contain default global usings that can be disabled by an MSBuild property.

[jonpryor]

As per #6118 (comment), I'm starting to wonder if anything should be added to @(Import) for Android apps. It feels like anything we add would be likely to introduce type resolution conflicts for MAUI and BCL types.