Directions crash when accessToken is not provided - it should use token from MGLMapboxAccessToken

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### Steps to reproduce

1. Set token using `MGLAccountManager.accessToken = "123123"`
2. Create object `Directions()`

### Expected behavior

Directions should be properly initialized with a token set using `MGLAccountManager.accessToken` if not provided via parameter or info.plist

### Actual behavior

There is a crash. Check this internal implementation of `init`:
```
public init(accessToken: String?, host: String?) {
let accessToken = accessToken ?? defaultAccessToken
precondition(accessToken != nil && !accessToken!.isEmpty, "A Mapbox access token is required. Go to . In Info.plist, set the MGLMapboxAccessToken key to your access token, or use the Directions(accessToken:host:) initializer.")
...
}
```

it uses `accessToken` from parameter or `defaultAccessToken` which is the one from `info.plist`. However, it is not using `MGLAccountManager.accessToken`.

The issue points to Directions.init(accessToken:host:) and its defaultAccessToken fallback; start there and inspect how MGLAccountManager.accessToken is exposed. Reproduce with MGLAccountManager.accessToken set and Directions() without a parameter, then verify initialization succeeds using that token.

Written by the indexing model from the issue text.