[build] Fix tools//<file> paths in Microsoft.Android.Sdk pack (#11689)

When inspecting `Microsoft.Android.Sdk.Windows.*.nupkg`, every file
that lives directly under `tools/` (e.g. `r8.jar`, `aapt2.exe`,
`bundletool.jar`, `Xamarin.Android.Build.Tasks.dll`, all the targets
files, etc.) is packed with a malformed entry path containing a
double slash:

    tools//r8.jar
    tools//aapt2.exe
    tools//bundletool.jar
    ...

83 of the 281 entries in the package are affected.

The cause is in `build-tools/create-packs/Microsoft.Android.Sdk.proj`:

```xml
<AndroidSdkBuildTools Include="@(MSBuildItemsWin)"
    TargetPath="tools\$([System.IO.Path]::GetDirectoryName('%(MSBuildItemsWin.RelativePath)'))" ... />
```

For an item whose `RelativePath` is just a filename like `r8.jar`,
`Path.GetDirectoryName('r8.jar')` returns `""`, so `TargetPath`
becomes `tools\` (trailing slash). The `SharedFramework.Sdk`
packaging logic then appends `/r8.jar`, producing `tools//r8.jar`.

Items with a real subdirectory (e.g. `cs\Foo.resources.dll`,
`bcl\Foo.dll`) are unaffected, because the directory name suppresses
the trailing-slash issue.

Switch to `Path.Combine`, which collapses an empty second argument
cleanly:

| Input `RelativePath`       | Old result    | New result    |
| -------------------------- | ------------- | ------------- |
| `r8.jar`                   | `tools\` ❌   | `tools` ✅    |
| `cs\Foo.resources.dll`     | `tools\cs` ✅ | `tools\cs` ✅ |
| `bcl\Foo.dll`              | `tools\bcl` ✅| `tools\bcl` ✅|

This bug has been latent since `dc089720b [build] Use
SharedFramework.Sdk to create workload packs (#10000)`.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

build-tools/create-packs/Microsoft.Android.Sdk.proj

@@ -42,8 +42,10 @@ core workload SDK packs imported by WorkloadManifest.targets.
       DependsOnTargets="ConstructInstallerItems;_GetLicense"
       BeforeTargets="GetFilesToPackage" >
     <ItemGroup>
-      <AndroidSdkBuildTools Include="@(MSBuildItemsWin)"  TargetPath="tools\$([System.IO.Path]::GetDirectoryName('%(MSBuildItemsWin.RelativePath)'))"  Condition=" '$(HostOS)' == 'Windows' " />
-      <AndroidSdkBuildTools Include="@(MSBuildItemsUnix)" TargetPath="tools\$([System.IO.Path]::GetDirectoryName('%(MSBuildItemsUnix.RelativePath)'))" Condition=" '$(HostOS)' == 'Linux' or '$(HostOS)' == 'Darwin' " />
+      <!-- Use Path.Combine so files at the root of MSBuildItems* (RelativePath has no directory)
+           produce 'tools' rather than 'tools\', which would otherwise be packed as 'tools//<file>'. -->
+      <AndroidSdkBuildTools Include="@(MSBuildItemsWin)"  TargetPath="$([System.IO.Path]::Combine('tools', $([System.IO.Path]::GetDirectoryName('%(MSBuildItemsWin.RelativePath)'))))"  Condition=" '$(HostOS)' == 'Windows' " />
+      <AndroidSdkBuildTools Include="@(MSBuildItemsUnix)" TargetPath="$([System.IO.Path]::Combine('tools', $([System.IO.Path]::GetDirectoryName('%(MSBuildItemsUnix.RelativePath)'))))" Condition=" '$(HostOS)' == 'Linux' or '$(HostOS)' == 'Darwin' " />
     </ItemGroup>
 
     <GenerateUnixFilePermissions