onBack stream not closed

## System info

Issue occurs on: both
Plugin version: 0.3.11
Flutter doctor output:

```
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.12.13+hotfix.9, on Microsoft Windows [Version 10.0.18363.900], locale en-NZ)

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
[√] Android Studio (version 4.0)
[√] VS Code, 64-bit edition (version 1.46.0)
[√] Connected device (1 available)

• No issues found!
```

## Steps to Reproduce
1. Navigate to the file base.dart in the plugin lib.
2. scroll down to dispose method.


as you can see also in the image, the _onBack stream controller is created but never closed in the dispose. Not sure if this was on purpose or not.

lib/base.dart を開いて dispose メソッドを調べ、そこで作成される _onBack ストリームコントローラーを追跡します。そのコントローラーを dispose 時に閉じるべきか確認し、プラグインが dispose されたときに onBack ストリームが閉じられることを検証します。

索引モデルが issue の本文から書いたものです。