[XABT] Fix build errors caused by enabling nullable checks in the tasks

[grendello]

Context: #10099
Context: #9918

#10099 enabled NRT checks for a number of XABT Tasks and was merged because all the tests passed. However, #10099 was not rebased on the tip of main which, in the meantime, contained code from the recently merged #9918 PR. #9918 introduced a couple of string properties to the AndroidComputeResPaths and CreateAar tasks, which were not nullable annotated.

This resulted in the following build errors once #10099 was merged:

src/Xamarin.Android.Build.Tasks/Tasks/AndroidComputeResPaths.cs(52,17): error CS8618: Non-nullable property 'PrefixProperty' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

src/Xamarin.Android.Build.Tasks/Tasks/CreateAar.cs(37,17): error CS8618: Non-nullable property 'PrefixProperty' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

This PR makes the PrefixProperty properties [Required] (because of the way they are used in the tasks) and assigns default values to them, as per convention introduced in #10099

[jonathanpeppers]

Merging to fix our build, don't think we need to rerun the test lanes.

[jpobst]

Thanks for getting this fixed!