PlayerUIList: make UI list private

Author: Profpatsch

app/src/main/java/org/schabi/newpipe/player/ui/PlayerUiList.kt

@@ -4,7 +4,7 @@ import org.schabi.newpipe.util.GuardedByMutex
 import java.util.Optional
 
 class PlayerUiList(vararg initialPlayerUis: PlayerUi) {
-    var playerUis = GuardedByMutex(mutableListOf<PlayerUi>())
+    private var playerUis = GuardedByMutex(mutableListOf<PlayerUi>())
 
     /**
      * Creates a [PlayerUiList] starting with the provided player uis. The provided player uis