Make LAYER_ID of BuildingPlugin public

This is helpful so other tools can reference it as an argument in `layerBelow` or `layerAbove`

Author: snkashis

plugin-building/src/main/java/com/mapbox/mapboxsdk/plugins/building/BuildingPlugin.java

@@ -42,7 +42,7 @@
  */
 public final class BuildingPlugin {
 
-  private static final String LAYER_ID = "mapbox-android-plugin-3d-buildings";
+  public static final String LAYER_ID = "mapbox-android-plugin-3d-buildings";
 
   private FillExtrusionLayer fillExtrusionLayer;
   private boolean visible;