Support JSpecify "nullness" annotations.

[jpobst]

Context: dotnet/android-libraries#1136
Context: https://developer.android.com/jetpack/androidx/releases/core#1.16.0

In version 1.16 of AndroidX "core", Google switched from using their androidx.annotation.NonNull types to new "industry-standard" "JSpecify nullness annotations". (Documentation)

Because generator does not support these annotations, all types that were previously "not-null" now appear to be "nullable".

We will need to extend generator to understand these new annotations to generate correct NRT code, as this is potentially just the beginning of Google migrating the entire AndroidX ecosystem to use them.

Note that these new annotations appear more complex than what we currently support, as you can mark a module/package/class/method as @NullMarked which means all types in that scope are considered "not-null" unless otherwise annotated. Scopes below a @NullMarked scope can additionally be marked as @NullUnmarked which undoes @NullMarked for the annotated scope.

JSpecify Nullness User Guide

[jonpryor]

Related: 07d5595, which adds parsing of Module information.

We may also need to add support for parsing package-info.class; Java allows packages to have annotations, which @NullMarked supports, a'la:

// src/example/package-info.java
@NullMarked
package example;

It's an otherwise "normal" .class file, with ThisClass being example/package-info. There could be a package-info.java for each separate package; there is no "inheritance", e.g. annotations on package example do not impact package example.nested.