MGLAccountManager.load method is slow during app startup

[pilot34]

I'm trying to optimize Booking app startup time. And noticed, that Mapbox is taking up to 100 ms because of the calls NSBundle.mainBundle in MGLAccountManager +load method.

NSBundle.mainBundle is slow for the first call and we can do that in a background thread after app is launched. But load methods are called before our code is called so I cannot optimize this place.

I guess we don't really need tokens in load method. The easiest solution will be to replace + load method with + initialize method that will be called only when we call MGLAccountManager for the first time.

The proper solution will be to move NSBundle calls to a background queue and load it asynchronously before we really need the token. And synchronize access to the token from the main thread somehow. Maybe with the same serial queue, or with read/write locks.

Steps to reproduce

1. Build any app with release configuration
2. Launch "Profile" in Xcode
3. Launch "App Launch" instrument

Expected behavior

Main thread is not blocked during launch. We can pre-warm NSBundle.mainBundle in background thread, and after that call MGLAccountManager and this won't block main thread.

Actual behavior

Main thread is blocked for around 80ms during launch because we call NSBundle.mainBundle in + load method.

Configuration

Mapbox SDK versions:
6.3.0
iOS/macOS versions:
14.3
Device/simulator models:
iPhone X
Xcode version:
12.3