[Code health] Convert all `@Singleton`s to Kotlin `object`s

[scolsen]

Kotlin has nice native support for declaring singletons using object declarations. Unfortunately, Dagger does not support injecting classes into objects, which at the moment pretty much prevents us from using an object for any construct that needs to depend on classes we'd normally inject.

If we convert all the classes we've currently marked as @Singleton into objects we can drop the several injections which should make kotlin code easier to refactor.