[Kotlin] Make ApiClient in jvm-retrofit2 be able to use own OkHttpClient

[tgerth]

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similar to PR: #6699

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[wing328]

cc @jimschubert (2017/09), @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

[4brunu]

@tgerth we don't need a new constructor, you can add the private val okHttpClient : OkHttpClient? = null to the constructor at line 18, because it has a default value.
And with this change, you can remove this constructor, and the var in line 49.

[wing328]

The commit history contains many changes (commits) not related to this PR. Better create a new PR by cherry-picking the commits related to Kotlin.