A gist for the article on FirebaseRemote config https://medium.com/p/89687a78e367

AdMobBannerData.kt

package com.leodroidcoder.remoteconfigsample

import com.google.gson.annotations.SerializedName

data class AdMobBannerData(
        @SerializedName("id") val id: String,
        @SerializedName("enabled") val enabled: Boolean
)

Extensions.kt

package com.leodroidcoder.remoteconfigsample

import com.google.gson.Gson

fun <T> Gson.jsonToObjectOrNull(json: String?, clazz: Class<T>): T? =
    try {
        fromJson(json, clazz)
    } catch (ignored: Exception) {
        null
    }

IRemoteConfigAPI.kt

package com.leodroidcoder.remoteconfigsample

interface IRemoteConfigAPI {

    fun getSomeCoolParameter(): String

    fun getBannerData(): AdBannerData

}

RemoteConfigManager.kt

package com.leodroidcoder.remoteconfigsample

import com.google.firebase.remoteconfig.FirebaseRemoteConfig
import com.google.firebase.remoteconfig.FirebaseRemoteConfigSettings
import com.google.gson.Gson
import javax.inject.Inject
import javax.inject.Singleton

@Singleton
class RemoteConfigManager @Inject constructor(
        private val gson: Gson
) : IRemoteConfigAPI {

    private val config: FirebaseRemoteConfig = FirebaseRemoteConfig.getInstance().apply {
        val configSettings = FirebaseRemoteConfigSettings.Builder()
                .setMinimumFetchIntervalInSeconds(CONFIG_CACHE_EXPIRATION_SECONDS)
                .build()
        setConfigSettingsAsync(configSettings)
        fetch(CONFIG_CACHE_EXPIRATION_SECONDS)
                .addOnCompleteListener {
                    if (it.isSuccessful) {
                        activateFetched()
                    }
                }
    }

    override fun getSomeCoolParameter(): String =
            read<String>(ConfigParam.SOME_COOL_PARAMETER) ?: SOME_DEFAULT_VALUE

    override fun getBannerData(): AdBannerData =
            read<AdBannerData>(ConfigParam.BANNER_DATA) ?: AdBannerData("manage_it_yourself", false)

    private inline fun <reified T> read(param: ConfigParam): T? = read(param, T::class.java)

    private fun <T> read(param: ConfigParam, returnType: Class<T>): T? {
        val value: Any? = when (returnType) {
            String::class.java -> config.getString(param.key)
            Boolean::class.java -> config.getBoolean(param.key)
            Long::class.java -> config.getLong(param.key)
            Int::class.java -> config.getLong(param.key).toInt()
            Double::class.java -> config.getDouble(param.key)
            Float::class.java -> config.getDouble(param.key).toFloat()
            else -> {
                val json = config.getString(param.key)
                json.takeIf { it.isNotBlank() }?.let { gson.jsonToObjectOrNull(json, returnType) }
            }
        }
        @Suppress("UNCHECKED_CAST")
        return (value as? T)
    }

    private enum class ConfigParam(val key: String) {
        SOME_COOL_PARAMETER("some_cool_parameter"),
        BANNER_DATA("banner_data"),
    }

    private companion object {
        /**
         * Config expiration interval 30 minutes.
         */
        private const val CONFIG_CACHE_EXPIRATION_SECONDS = 900L
        private const val SOME_DEFAULT_VALUE = "Any default value"
    }
}