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)
```

Reproduce the warning on Android with flutter_webview_plugin 0.3.8 using the reported setup, starting when the screen appears. Inspect the plugin's native Java/JNI handling and use the repeated ART log as the failure signal. Done means the warning no longer prints continuously when the WebView opens.

Written by the indexing model from the issue text.