[jnigen] Remove TypeUsage indirection

[HosseinYousefi]

Closes #2010.

Previously we had a TypeUsage class that really didn't do anything other than nest a ReferredType because json_serializable doens't support union types (as ReferredType could be PrimtiveType, DeclaredType, ...).

This is unnecessary as we can simply parse that level manually instead of relying on json_serializable's codegen.

Unrelated change: more descriptive doc comments that includes the original type of methods and fields including their generic types.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[HosseinYousefi]

The ubuntu beta test failure is not related to this PR, the sources are not changed other than doc comments.

Opened another issue for this flake: #2188