[xamarin-android-tools] import $(LibZipSharpVersion) value (dotnet#8738)

Fixes: dotnet/maui#20752

Context: d1ba2cc
Context: dotnet/android-tools@34e98e2
Context: dotnet#8746

`dotnet build` (.NET Core) ignores assembly versions.
`msbuild.exe` (.NET Framework) does not.

Enter d1ba2cc, which updates the `libZipSharp.dll` assembly version
from 3.0.0.0 to 3.1.1.0.  However, this change only impacted
`Xamarin.Android.Build.Tasks.dll`:

	% monodis --assemblyref bin/Release/lib/packs/Microsoft.Android.Sdk.Darwin/34.99.0/tools/Xamarin.Android.Build.Tasks.dll
	…
	11: Version=3.1.1.0
		Name=libZipSharp
		Flags=0x00000000

It did *not* impact `Microsoft.Android.Build.BaseTasks.dll`, as it
is built by `external/xamarin-android-tools` and was using a
`@(PackageReference)` for libZipSharp 3.0.0:

	% monodis --assemblyref bin/Release/lib/packs/Microsoft.Android.Sdk.Darwin/34.99.0/tools/Microsoft.Android.Build.BaseTasks.dll
	…
	5: Version=3.0.0.0
	        Name=libZipSharp
	        Flags=0x00000000

The resulting NuGet package only contains *one* `libZipSharp.dll`,
the 3.1.1.0 version (what `Xamarin.Android.Build.Tasks.dll` refs).

On PR builds and CI, everything was fine, because all the tests we
have use `dotnet build`, and .NET Core ignores assembly versions.

However, if you use *`msbuild.exe`* to invoke the tasks within
`Microsoft.Android.Build.BaseTasks.dll`, things fail:

	XARLP7028 System.IO.FileNotFoundException: Could not load file or assembly 'libZipSharp, Version=3.0.0.0, Culture=neutral, PublicKeyToken=276db85bc4e20efc' or one of its dependencies.
	The system cannot find the file specified.File name: 'libZipSharp, Version=3.0.0.0, Culture=neutral, PublicKeyToken=276db85bc4e20efc'
	   at Xamarin.Android.Tasks.ResolveLibraryProjectImports.Extract(IDictionary`2 jars, ICollection`1 resolvedResourceDirectories, ICollection`1 resolvedAssetDirectories, ICollection`1 resolvedEnvironments, ICollection`1 proguardConfigFiles)
	   at Xamarin.Android.Tasks.ResolveLibraryProjectImports.RunTask()
	   at Microsoft.Android.Build.Tasks.AndroidTask.Execute() in /Users/runner/work/1/s/xamarin-android/external/xamarin-android-tools/src/Microsoft.Android.Build.BaseTasks/AndroidTask.cs:line 25

Fix this by adding an `external/xamarin-android-tools.override.props`
file (dotnet/android-tools@34e98e2b) which imports
`Directory.Build.props`, which causes `$(LibZipSharpVersion)` to be
set so that the xamarin-android-tools build uses values provided by
xamarin-android.

TODO: dotnet#8746 adds a unit test for the
"build with `msbuild.exe`" scenario.

external/xamarin-android-tools.override.props

@@ -0,0 +1,4 @@
+<Project>
+  <!-- Import $(LibZipSharpVersion) -->
+  <Import Project="..\Directory.Build.props" />
+</Project>