…values

There is an email thread about ASP.NET projects in VS -- and something
is potentially slowing down builds when optional workloads are
installed.

One concern is our `AutoImport.props`, take for instance the following
`foo.targets` file:

    <Project>
      <ItemGroup>
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.xml" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.axml" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.png" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.jpg" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.gif" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.webp" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.ttf" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.otf" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.ttc" />
        <AndroidResource Include="$(MonoAndroidResourcePrefix)\raw\*" Exclude="$(MonoAndroidResourcePrefix)\raw\.*" />
        <AndroidAsset Include="$(MonoAndroidAssetsPrefix)\**\*" Exclude="$(MonoAndroidAssetsPrefix)\**\.*\**" />
      </ItemGroup>
      <Target Name="Build" />
    </Project>

In this case `$(MonoAndroidResourcePrefix)` and
`$(MonoAndroidAssetsPrefix)` will be blank, and we get:

    msbuild foo.targets -bl
    ...
      MSBUILD : warning MSB5029: The value "\**\.*\**" of the "Exclude" attribute in element <ItemGroup> in file "foo.targets (13,61)" is a wildcard that results in enumerating all files on the drive, which was likely not intended. Check that referenced properties are always defined.
    1 Warning(s)
    0 Error(s)
    Time Elapsed 00:02:45.14

I'm not sure this is causing an actual problem, but I think it's a
good idea to add some safety here.

I will make a similar change in Xamarin.Legacy.Sdk.