fixed race condition with map#waitForMap

[felix-ht]

Multiple attempts have been made to fix the the issue that sometimes on iOS onStyleLoaded has does not fire. #775
However the error still seems to show up for some users - albeit very rarely.

With this PR im pretty confident to truly fix the issue.
The root cause seems to be that on the native side onStyleLoaded fires before the flutter code has even setup the platform callbacks. This causes mainly two kinds of issues:

• onMapCreated and onStyleLoaded being called out of order
• onStyleLoaded not being called at all

To fix it i changed the implementation so that map#waitForMap calls onStyleLoaded if on the map has already finished loading the style. If map#waitForMap happens first onStyleLoaded is called in the mapView callback.

On subsequent style changes mapView always directly calls onStyleLoaded.

@AAverin it would be great if you could check this code with your test code as well - to make sure that is gone for you as well.

[AAverin]

@felix-ht would it also help with #807?
I had to introduce 100ms delay on iOS for camera to work somewhat reliably.

[felix-ht]

@AAverin There is a good chance thats this solves that issue that as well. Would be great if you could test!

[AAverin]

@felix-ht Yes, I will try to test tomorrow against my codebase. Android was quite stable, but there were many issues on iOS

[AAverin]

@felix-ht Doesn't help with the animation issue I had, will do some more tests

[AAverin]

@felix-ht After all the fixes I did before onStyleLoaded is invoked quite stable for me, maybe I am just not catching some edge case. The only big issue I have is that calling camera with animation inside onStyleLoaded doesn't work.
This issue is not resolved with this PR and the only workaround that worked for me is to add 100ms delay before calling any methods on the controller.
This is probably related to some native implementation because on Android everything works fine without delays.

If you say that this PR resolves some minor edge case, let's merge it.