Attempt to remove non-JNI local reference, dumping thread

## System info

Issue occurs on: tested only on Android
Plugin version: 0.3.8
Flutter doctor output:

```
[✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Linux, locale pt_BR.UTF-8)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[!] Android Studio (not installed)
[✓] VS Code (version 1.38.1)
[✓] Connected device (1 available)
```

## Steps to Reproduce

1. when screen shows up

## Logs

```
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
I/art ( 2809): Background sticky concurrent mark sweep GC freed 29(752B) AllocSpace objects, 0(0B) LOS objects, 0% free, 13MB/13MB, paused 5.379ms total 7.415ms
```
and keep printing it in an infinite loop...

```
✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Linux, locale pt_BR.UTF-8)
• Flutter version 1.9.1+hotfix.2 at /home/rodrigo/Development/flutter
• Framework revision 2d2a1ffec9 (2 weeks ago), 2019-09-06 18:39:49 -0700
• Engine revision b863200c37
• Dart version 2.5.0

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
• Android SDK at /home/rodrigo/Android/SDK
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.3
• ANDROID_HOME = /home/rodrigo/Android/SDK
• Java binary at: /usr/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_222-8u222-b10-1~deb9u1-b10)
• All Android licenses accepted.

[!] Android Studio (not installed)
• Android Studio not found; download from https://developer.android.com/studio/index.html
(or visit https://flutter.dev/setup/#android-setup for detailed instructions).

[✓] VS Code (version 1.38.1)
• VS Code at /usr/share/code
• Flutter extension version 3.4.1

[✓] Connected device (1 available)
• Custom Phone • 192.16[...]:5555 • android-x86 • Android 6.0 (API 23)
```

報告されたセットアップを使用し、画面が表示された時点から flutter_webview_plugin 0.3.8 で Android 上の警告を再現します。プラグインのネイティブな Java/JNI 処理を調査し、繰り返される ART ログを失敗のシグナルとして使用します。WebView を開いたときに警告が継続的に出力されなくなれば完了です。

索引モデルが issue の本文から書いたものです。