remove @Keep annotation, and enhance proguard rules for release version

android/consumer-proguard-rules.pro

@@ -1,15 +1,18 @@
 -keep class com.linecorp.flutter_line_sdk.** { *; }
 -keepattributes Signature
 -keepattributes *Annotation*
+
 ## gson
 -dontwarn sun.misc.**
 -keep class com.linecorp.flutter_line_sdk.model.** { *; }
+-keepclassmembers class com.linecorp.flutter_line_sdk.model.** { <fields>; }
+-keepclassmembers class com.linecorp.linesdk.** { <fields>; }
 -keep class * implements com.google.gson.TypeAdapter
 -keep class * implements com.google.gson.TypeAdapterFactory
 -keep class * implements com.google.gson.JsonSerializer
 -keep class * implements com.google.gson.JsonDeserializer
 
 # Prevent R8 from leaving Data object members always null
--keepclassmembers,allowobfuscation class * {
+-keepclassmembers class * {
   @com.google.gson.annotations.SerializedName <fields>;
 }

android/src/main/kotlin/com/linecorp/flutter_line_sdk/model/AccessToken.kt

@@ -1,12 +1,10 @@
 package com.linecorp.linesdk.unitywrapper.model
 
-import android.support.annotation.Keep
 import com.google.gson.Gson
 import com.google.gson.annotations.SerializedName
 import com.linecorp.linesdk.auth.LineLoginResult
 
 
-@Keep
 data class AccessToken(
     @SerializedName("access_token")
     val accessToken: String,

android/src/main/kotlin/com/linecorp/flutter_line_sdk/model/BotFriendshipStatus.kt

@@ -1,7 +1,5 @@
 package com.linecorp.linesdk.unitywrapper.model
 
-import android.support.annotation.Keep
-
-
-@Keep
-data class BotFriendshipStatus(val friendFlag: Boolean)
+data class BotFriendshipStatus(
+    val friendFlag: Boolean
+)

android/src/main/kotlin/com/linecorp/flutter_line_sdk/model/Error.kt

@@ -1,9 +1,6 @@
 package com.linecorp.linesdk.unitywrapper.model
 
-import android.support.annotation.Keep
 
-
-@Keep
 data class Error(
     val code: Int,
     val message: String

android/src/main/kotlin/com/linecorp/flutter_line_sdk/model/LoginResultForFlutter.kt

@@ -1,11 +1,9 @@
 package com.linecorp.linesdk.unitywrapper.model
 
-import android.support.annotation.Keep
 import com.linecorp.flutter_line_sdk.model.UserProfile
 import com.linecorp.linesdk.Scope
 import com.linecorp.linesdk.auth.LineLoginResult
 
-@Keep
 data class LoginResultForFlutter(
     val accessToken: AccessToken,
     val scope: String,

android/src/main/kotlin/com/linecorp/flutter_line_sdk/model/UserProfile.kt

@@ -1,9 +1,7 @@
 package com.linecorp.flutter_line_sdk.model
 
-import android.support.annotation.Keep
 import com.linecorp.linesdk.LineProfile
 
-@Keep
 data class UserProfile(
     val userId: String,
     val displayName: String,

android/src/main/kotlin/com/linecorp/flutter_line_sdk/model/VerifyAccessTokenResult.kt

@@ -1,9 +1,7 @@
 package com.linecorp.linesdk.unitywrapper.model
 
-import android.support.annotation.Keep
 import com.google.gson.annotations.SerializedName
 
-@Keep
 data class VerifyAccessTokenResult(
     @SerializedName("client_id")
     val channelId: String,