Unable to override style for main route in NavigationViewController when routeLineTracksTraversal is set to true.

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**Mapbox Navigation SDK version:** `1.1.0`

### Steps to reproduce

Implement such `NavigationViewControllerDelegate` method:
```swift
func navigationViewController(_ navigationViewController: NavigationViewController, mainRouteStyleLayerWithIdentifier identifier: String, source: MGLSource) -> MGLStyleLayer? {
let layer = MGLLineStyleLayer(identifier: identifier, source: source)
layer.predicate = NSPredicate(format: "isAlternateRoute == false")
layer.lineColor = NSExpression(forConstantValue: #colorLiteral(red: 0.4666666687, green: 0.7647058964, blue: 0.2666666806, alpha: 1))
layer.lineWidth = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'linear', nil, %@)", MBRouteLineWidthByZoomLevel.multiplied(by: 0.8))
layer.lineJoin = NSExpression(forConstantValue: "round")
layer.lineCap = NSExpression(forConstantValue: "miter")

return layer
}
```
Set `NavigationViewController.routeLineTracksTraversal = true`, start active guidance navigation and observe behavior.

### Expected behavior

`NavigationViewController` should retain any modifications which were added to main route line during active guidance. Since `MGLLineStyleLayer.lineColor` property is set to custom color it should be used instead of default congestion levels.

### Actual behavior

`NavigationViewController` overwrites modifications which were applied to main route/casing during active guidance.

Start at the NavigationViewControllerDelegate method mainRouteStyleLayerWithIdentifier and follow how routeLineTracksTraversal updates the main route and casing during active guidance. Reproduce the provided setup with a custom lineColor, then verify that the custom style remains in effect instead of being replaced by default congestion styling.

Written by the indexing model from the issue text.