Fix Memory Leaks on iOS

[kuglemic]

This PR fixes a memory leak on iOS using Apple Maps where AIRMap didn't get deallocated if the component is unmounted during an regionChange animation.

Does any other open PR do the same thing?

No

What issue is this PR fixing?

Didn't file an issue for it and haven't found one.

How did you test this PR?

I did test it on the iPhone 8 Simulator and on a real iPhone 7 device.

Steps to reproduce:

1. Create a simple Test App where you can mount and unmount the Map Component.
2. Mount the Map component
3. Change the region of the map animated, e.g. by calling: animateToRegion
4. Make sure that - (void)mapView:(AIRMap *)mapView regionWillChangeAnimated:(__unused BOOL)animated gets called. This method did cause the leak.
5. Unmount the Map component during the animation, this means before - (void)mapView:(AIRMap *)mapView regionDidChangeAnimated:(__unused BOOL)animated did get called.
6. Notice that dealloc didn't get called in AIRMap.m
7. If you Mount the Map component again, you will get a new instance of AIRMap, which causes a memory increase of ~30MB in my simulator

[alvelig]

These are the PR we are missing. Thank you. LGTM @rborn 🐽