[jni] Fix the context when using multiple flutter engines

[HosseinYousefi]

Uses a map on the Java side to store all the plugin instances with the key of engineId. On the Dart side, we use the engineId to access to correct Flutter Engine's context even when there are multiple.

Fetching activity is now exposed via a stream that users will listen to instead of a single cached one. Flutter users will have to use a StreamBuilder or similar to access activity this way.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
jni	Breaking	0.14.2	0.15.0-wip	0.15.0	✔️

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License Headers ✔️

// 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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[coveralls]

coverage: 86.562% (-5.4%) from 91.972%
when pulling 1c04bd4 on fix-context
into f7c7d52 on main.