fix(location): update ios location implementation to handle thread issues

[CoryWritesCode]

We've been getting a steady stream of EXC_BAD_ACCESS crashes in LocationModuleImpl showing up on _dispatch_worker_thread2. Turns out CLLocationManager is thread-affine — you have to talk to it on the thread you created it on. getCurrentPosition: creates the manager on main, but locationManagerDidChangeAuthorization: was hopping onto a background queue and poking the manager from there. iOS 14+ really doesn't like that.

The original reason for the background hop was fine: +locationServicesEnabled isn't supposed to run on main because it can block. The fix just took too much stuff off-main along for the ride.

Fix

• locationManagerDidChangeAuthorization: now grabs authorizationStatus on the delegate thread, hops to background only for +locationServicesEnabled, then comes back to main before touching the manager. Moved the actual handling into a new handleAuthorizationStatus:servicesEnabled: helper.
• clearReferences bounces back to main if it's called off-main, and nils out the delegate before stopUpdatingLocation so a late callback can't land on a dead promise.
• Added the mReject != nil check in the two spots it was missing.
• Turned the file-scope statics (mTimer / mLocationManager / mResolve / mReject / mTimeout) into ivars. No behavior change, just less scary.
• isAuthorized returns NO if the manager is nil instead of messaging nil.