Fix race on set route

[korshaknn]

Description

closes #4753

Changelog

<changelog>Fixed an occasional crash when setting a new route while on a non-initial leg of the previous route.</changelog>

Screenshots or Gifs

[LukasPaczos]

🚀

Could you add a unit test that verifies that the route reference is only set after the async job finishes?

[LukasPaczos]

We could now be running into a problem where the RoutesObserver will immediately fire with the new route, while RouteProgress#route still holds the old route. We don't know how developers are interacting with these API and I'm sure there are some assumptions that they are always equal, and those are probably rightful assumptions. Wouldn't it be safer to somehow pause route progress updates while we're waiting for the native set route job to finish?

[korshaknn]

@LukasPaczos tests added, RouteProgress issue fixed

[LukasPaczos]

LGTM. This introduces a lag to location updates while the route is updating but shouldn't be too impactful in most of the scenarios, while we'll probably resolve it with async native setRoute in the future. This all is also related to #4727 since now we're missing even more events.