Local style source file example

[cammace]

Now that mapbox/mapbox-gl-native#6497 it makes it possible to store the style JSON file locally.

cc: @zugaldia

[cammace]

Similar to #163, let's just find an existing style and load the map style into the example from a local json file.

[zugaldia]

Would it make sense for this example to take advantage of mapbox/mapbox-plugins-android#38 too?

[langsmith]

Fyi @cammace , I'm planning to use Wham! in this example which will mean that we have Wham!, Standard, and Moonlight in the demo app and can close #163