Error on Android:Failed resolution of: Lcom/google/gson/Gson;

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## Is it a security issue?
No
## What did you do?
Unity 2018.4.7f1
>1 Import line_sdk_unity_1.1.4.unitypackage to a empty unity project.
>2 Switch platform to Andorid.
>3 open "Main" scene and replace Channel ID on LineSDK object.
>4 Build apk with mini api level21.
>5 Run apk in sumsung SM-G9600

## What did you expect?

The build Run with no error

## What happened actually?

```
2020-09-07 11:29:50.762 5604-5624/? E/Unity: AndroidJavaException: java.lang.NoClassDefFoundError: Failed resolution of: Lcom/google/gson/Gson;
java.lang.NoClassDefFoundError: Failed resolution of: Lcom/google/gson/Gson;
at com.linecorp.linesdk.unitywrapper.LineSdkWrapper.(LineSdkWrapper.kt:21)
at com.unity3d.player.UnityPlayer.nativeRender(Native Method)
at com.unity3d.player.UnityPlayer.c(Unknown Source:0)
at com.unity3d.player.UnityPlayer$e$2.queueIdle(Unknown Source:72)
at android.os.MessageQueue.next(MessageQueue.java:395)
at android.os.Looper.loop(Looper.java:181)
at com.unity3d.player.UnityPlayer$e.run(Unknown Source:32)
Caused by: java.lang.ClassNotFoundException: Didn't find class "com.google.gson.Gson" on path: DexPathList[[zip file "/data/app/com.chm.linetest-letwctzDNm2HWtnsrXKO3A==/base.apk"],nativeLibraryDirectories=[/data/app/com.chm.linetest-letwctzDNm2HWtnsrXKO3A==/lib/arm, /data/app/com.chm.linetest-letwctzDNm2HWtnsrXKO3A==/base.apk!/lib/armeabi-v7a, /system/lib]]
at dalvik.system.BaseDexClassLoader.findClass(BaseDexClas
```

## Your environment?

Build from 2018.4.7f1
LineSDK 1.1.4

## Sample project

https://drive.google.com/file/d/1wvDOGwkAolomlllUePsXElTznq4VCu3O/view?usp=sharing

Reproduce the Android build from the Unity 2018.4.7f1 steps using line_sdk_unity_1.1.4 and the linked sample project. Start with the LineSdkWrapper constructor at LineSdkWrapper.kt:21 and inspect the generated APK; done means the app runs without the reported Gson NoClassDefFoundError.

Written by the indexing model from the issue text.