Set custom map style for drop-in navigation UI – *Work in progress*

[langsmith]

This pr adds the ability to set a custom map style URL for the map that's used in the drop-in nav UI.

I started this work mainly to have more UI consistency in the Store Locator kit.

Right now, I'm using @nullable and if/else statements, so there's probably more elegant/concise refactoring to be done.

Also, in case you want to try a map style in the drop-in nav UI yourself, pass through a style url string as the last parameter of the NavigationLauncher.startNavigation() method found in launchNavigationWithRoute() of the test app's
NavigationViewActivity.java.

Maybe we can pass through mapboxMap.getStyleUrl in some cases to automatically have UI consistency?

cc @danesfeder @cammace @zugaldia @Lndsay @noemiwalzebuck

[danesfeder]

Capturing conversation with @ericrwolfe - I think this PR is a good opportunity to open up "theming" for the drop-in UI.

This will include the map style work that @langsmith has started here, as well as the ability to set a custom theme with the custom attributes we have defined https://github.com/mapbox/mapbox-navigation-android/blob/master/libandroid-navigation-ui/src/main/res/values/attrs.xml#L15

I can work with @langsmith on the rest of this / come up with a solution that is easy and makes sense from a developer integration standpoint.

[danesfeder]

@langsmith this functionality landed in #507 - appreciate you getting the ball rolling on this! Docs will be updated to reflect how this can be done