Use a property instead of a getter function for key in BubbleBarBubble. Using...

Use a property instead of a getter function for key in BubbleBarBubble. Using a property here is more Kotlin idiomatic.

https://kotlinlang.org/docs/coding-conventions.html#functions-vs-properties
go/kotlin/practices/properties#when

Bug: 281083629
Test: Verified on a physical device
Change-Id: I4c003c6633bf1913ee5e1e86224a679ff3442e2b