Remove ArrivalObserver default implementations

[Guardiola31337]

Description

Removes ArrivalObserver default implementations

• I have added any issue links
• I have added all related labels (bug, feature, new API(s), SEMVER, etc.)
• I have added the appropriate milestone and project boards

Goal

ArrivalObserver is currently an interface with default implementations for onStopArrival and onRouteArrival, so because we're supporting Java 7 (as of now 😁), it'll be unnecessarily converted to an abstract class on the Java side. The idea behind this was that as a client you don't need to override both if you don’t want to e.g.

private val arrivalObserver = object : ArrivalObserver {
    override fun onRouteArrival(routeProgress: RouteProgress) {
        findViewById<Button>(R.id.navigateNextRouteLeg).visibility = View.GONE
    }
}

Only in Kolin though because in Java 7 you'd be forced to implement both. Interface segregation principle is another option to fix this.

We can revisit and bring this back when we support Java 8 (hopefully soon 😅).

Implementation

Remove default implementations ({}) from onStopArrival and onRouteArrival (ArrivalObserver)

Testing

• I have tested locally (including SNAPSHOT upstream dependencies if needed) through testapp/demo app and run all activities to avoid regressions
• I have tested via a test drive, or a simulation/mock location app
• New and existing unit tests pass locally with my changes

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have updated the CHANGELOG including this PR

cc @kmadsen @abhishek1508 @LukasPaczos @langsmith

[Guardiola31337]

cc @JunDai as you're working on #2928