neither is important to "know first" but Java is recommended since many apps and tutorials use Java and people haven't switched yet.
I say learn both, but start with Java, get more skills under your belt
When Android was came then Java was 1st official language of Android App Development but after some time Kotlin came in market and Google declared it as a "Preferred language" of Android App Development but Java is still officially supported.
So Java and Kotlin both may be use in Android App Development.
You can learn both.