[jnigen] migrate in_app_java example to Dart API

[goderbauer]

Follow-up to #2670.
Related to #1640.

Reworks the in_app_java example to use the Dart API to match the example given in the readme.

[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/objective_c/lib/src/ns_input_stream.dart

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[goderbauer]

Of course, I need to update CI. Will do so tomorrow.

[dcharkes]

Once CI is green, LGTM!

[dcharkes]

nit: sourcePath is singular, but the type is a list.

[dcharkes]

How often do we pass this or not? Are we passing this to 90% of our use cases? If yes, should this be the default? And if it's the default, does that require there always is a Gradle project configured? How is this Gradle project configured, always via flutter create?

[HosseinYousefi]

You either can download and pass the jars or use this option, I don't have data to say 90% or 80%.

Always via flutter create, yes.

[coveralls]

coverage: 81.228% (+0.4%) from 80.804%
when pulling a197397 on goderbauer:reworkjniex
into 0924cb0 on dart-lang:main.