Bump to dotnet/android-tools@46b5284 (#11521)

Bumps `external/xamarin-android-tools` from `7c05b5c` to `46b5284`.

## Commits

* dotnet/android-tools#369 - Update Microsoft.Build package references (17.14.28 → 18.6.3)
* dotnet/android-tools#370 - Add Dependabot config for weekly NuGet updates
* dotnet/android-tools#377 - [BaseTasks] Remove unused System.Reflection.Metadata reference

## Notes

The MSBuild 18.6.3 bump (#369) required matching updates here:

* `Directory.Build.targets` bumps `System.CodeDom` from `9.0.8` to `10.0.3` to satisfy the new transitive requirement from `Microsoft.Build.Tasks.Core 18.6.3`.

`Microsoft.Android.Build.BaseTasks` no longer pins `System.Reflection.Metadata` after android-tools#377, so downstream consumers (e.g. `android-platform-support`) are not forced onto the `11.0.0-preview` version this repo uses for `Microsoft.Android.Sdk.TrimmableTypeMap`.

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

Directory.Build.targets

@@ -4,7 +4,7 @@
   <ItemGroup>
     <PackageReference Update="Microsoft.SourceLink.GitHub"                  Version="8.0.0" />
     <PackageReference Update="Microsoft.Win32.Registry"                     Version="5.0.0" />
-    <PackageReference Update="System.CodeDom"                               Version="9.0.8" />
+    <PackageReference Update="System.CodeDom"                               Version="10.0.3" />
     <PackageReference Update="Irony"                                        Version="1.1.0" />
   </ItemGroup>