Use Kotlin-friend Java method names for Boolean accessors

[jparise]

From Calling Java code from Kotlin:

Boolean accessor methods (where the name of the getter starts with is and the name of the setter starts with set) are represented as properties which have the same name as the getter method.

This could be a nice improvement for our generated "is set" methods. We currently name them get{Property}IsSet() (e.g. getHeightIsSet()), but we could instead name them is{Property}Set() (e.g. isHeightSet()).

We could also consider doing something similar for declared Boolean property accessors, but we'd need to be more thoughtful about unexpected results if we start changing property names to match this convention.