[android] layer list accessor + example

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/MapboxMap.java

@@ -209,6 +209,22 @@ public final class MapboxMap {
     nativeMapView.setTransitionDelay(delay);
   }
 
+  /**
+   * Retrieve all the layers in the style
+   *
+   * @return all the layers in the current style
+   */
+  @UiThread
+  public List<Layer> getLayers() {
+    return nativeMapView.getLayers();
+  }
+
+  /**
+   * Get the layer by id
+   *
+   * @param layerId the layer's id
+   * @return the layer, if present in the style
+   */
   @Nullable
   @UiThread
   public Layer getLayer(@NonNull String layerId) {

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/NativeMapView.java

@@ -747,6 +747,13 @@ final class NativeMapView {
     nativeSetTransitionDelay(delay);
   }
 
+  public List<Layer> getLayers() {
+    if (isDestroyedOn("getLayers")) {
+      return null;
+    }
+    return Arrays.asList(nativeGetLayers());
+  }
+
   public Layer getLayer(String layerId) {
     if (isDestroyedOn("getLayer")) {
       return null;
@@ -1054,6 +1061,8 @@ final class NativeMapView {
 
   private native void nativeSetTransitionDelay(long delay);
 
+  private native Layer[] nativeGetLayers();
+
   private native Layer nativeGetLayer(String layerId);
 
   private native void nativeAddLayer(long layerPtr, String before) throws CannotAddLayerException;