[main][msbuild] Changes GetMinimumOSVersionTaskBase to use ITaskItem for inputs (#9135)

* [msbuild] Changes GetMinimumOSVersionTaskBase to use ITaskItem for inputs

From Visual Studio we need the input files to be  instead of plain strings to identify the files that need to be copied to the Mac

* Adds null check for AppManifest in GetMinimumOSVersionTaskBase

Co-authored-by: emaf <ema@xamarin.com>

msbuild/Xamarin.MacDev.Tasks.Core/Tasks/GetMinimumOSVersionTaskBase.cs

@@ -6,7 +6,7 @@ using Xamarin.Localization.MSBuild;
 
 namespace Xamarin.MacDev.Tasks {
 	public abstract class GetMinimumOSVersionTaskBase : XamarinTask {
-		public string AppManifest { get; set; }
+		public ITaskItem AppManifest { get; set; }
 
 		[Required]
 		public string SdkVersion { get; set; }
@@ -18,9 +18,9 @@ namespace Xamarin.MacDev.Tasks {
 		{
 			PDictionary plist = null;
 
-			if (!string.IsNullOrEmpty (AppManifest)) {
+			if (!string.IsNullOrEmpty (AppManifest?.ItemSpec)) {
 				try {
-					plist = PDictionary.FromFile (AppManifest);
+					plist = PDictionary.FromFile (AppManifest.ItemSpec);
 				} catch (Exception ex) {
 					Log.LogError (null, null, null, AppManifest, 0, 0, 0, 0, MSBStrings.E0010, AppManifest, ex.Message);
 					return false;