[Xamarin.Android.Build.Tasks] ApkSigner Failed if minsdk > maxSdk (#1003)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=60444

We should NOT ever pass a `//uses-sdk/@android:minSdkVersion` value
that is greater than the `//uses-sdk/@android:maxSdkVersion` value.
It results in the following error:

	Min API Level (10) > max API Level (8)

We should check the values and use the minimum value for
`//uses-sdk/@android:minSdkVersion`.

src/Xamarin.Android.Build.Tasks/Tasks/AndroidApkSigner.cs

@@ -56,6 +56,7 @@ namespace Xamarin.Android.Tasks
 			if (manifest.TargetSdkVersion.HasValue)
 				maxSdk = manifest.TargetSdkVersion.Value;
 
+			minSdk = Math.Min (minSdk, maxSdk);
 			cmd.AppendSwitch ("sign");
 			cmd.AppendSwitchIfNotNull ("--ks ", KeyStore);
 			cmd.AppendSwitchIfNotNull ("--ks-pass pass:", StorePass);