Bump to xamarin/xamarin-android-tools/main@26d65d9 (#741)

Changes: dotnet/android-tools@f2af06f...26d65d9

  * dotnet/android-tools@26d65d9: [Xamarin.Android.Tools.AndroidSdk] Update SDK component for API-30 (#99)
  * dotnet/android-tools@1878e43: [Xamarin.Android.Tools.AndroidSdk] Error & Warning Localization (#96)

Additionally, the "main" branch for xamarin/xamarin-android-tools has
changed from xamarin-android-tools/master to
xamarin-android-tools/main.  Update `.gitmodules` accordingly.

Finally, update the `nuget restore Xamarin.Android.Tools.sln` command
executed by `azure-pipelines.yaml` to also provide
`-ConfigFile external\xamarin-android-tools\NuGet.config` in order to
prevent the following failure on CI:

	NU1101: Unable to find package XliffTasks. No packages exist with this id in source(s): NuGetOrg

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "external/xamarin-android-tools"]
 	path = external/xamarin-android-tools
 	url = https://github.com/xamarin/xamarin-android-tools.git
-	branch = master
+	branch = main

build-tools/automation/azure-pipelines.yaml

@@ -39,8 +39,8 @@ jobs:
 
   - task: NuGetCommand@2
     inputs:
-      command: restore
-      restoreSolution: 'external\xamarin-android-tools\Xamarin.Android.Tools.sln'
+      command: custom
+      arguments: restore external\xamarin-android-tools\Xamarin.Android.Tools.sln -ConfigFile external\xamarin-android-tools\NuGet.config
 
   - task: MSBuild@1
     displayName: MSBuild Java.Interop.sln /t:Prepare