[Xamarin.Android.Build.Tasks] fix `<GenerateRtxt/>` during design-time builds

[jonathanpeppers]

Fixes: https://github.com/dotnet/android/issues/9285

Building a .NET 9 project without API 35 installed would fail with:

MSB4044: The "GenerateRtxt" task was not given a value for the required parameter "JavaPlatformJarPath"

I was able to reproduce this in an MSBuild test that uses:

var androidSdkPath = CreateFauxAndroidSdkDirectory (Path.Combine (path, "android-sdk"), "35.0.0", []);

To fix this:

• Remove [Required] from the JavaPlatformJarPath property

• Add some extra checks if the value is null or empty

I also did some other cleanup:

• MonoAndroidHelper now logs during the <GetJavaPlatformJar/> task, even during a design-time build.

• Removed what looked like an unused, extra line in FileResourceParser:

  string publicXmlPath = Path.Combine (JavaPlatformDirectory, "data", "res", "values");