[android] don't include .so files in class libraries

[jonathanpeppers]

Fixes: dotnet/android#5776
Fixes: dotnet/android#6545

If you do:

1. File -> New Xamarin.Android class library
2. Add <PackageReference Include="SkiaSharp" Version="2.80.3" />

You end up with 13MB .dll file with C# code inside! The class
library is redistributing the SkiaSharp native libraries.

$(ShouldIncludeNativeSkiaSharp) should default to False for class
libraries.

To test this change, I manually edited:

%userprofile%\.nuget\packages\skiasharp\2.80.3\build\monoandroid1.0\SkiaSharp.targets
%userprofile%\.nuget\packages\skiasharp\2.80.3\buildTransitive\monoandroid1.0\SkiaSharp.targets

Now my class library is 76,800 bytes.

When consuming this class library in an Android application project,
everything should still work as expected. However, the application
will need to reference SkiaSharp -- which is actually ideal.

API Changes

None

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Updated documentation

[jonathanpeppers]

/cc @mattleibow, looks like I don't have permission to add you as reviewer.

[mattleibow]

This is a good change because I basically did something like this for HarfBuzzSharp. Previously there was logic to include the native binaries, but was then updated to actually be a no-op - the .targets file is now empty.

In this case, we actually should have done the same and deleted the .targets file. However, we still need to preserve the functionality of the $(ShouldIncludeNativeSkiaSharp) property and make sure it is in the item group.