[android] source list accessor + example

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

@@ -291,6 +291,22 @@ public final class MapboxMap {
     nativeMapView.removeLayer(layer);
   }
 
+  /**
+   * Retrieve all the sources in the style
+   *
+   * @return all the sources in the current style
+   */
+  @UiThread
+  public List<Source> getSources() {
+    return nativeMapView.getSources();
+  }
+
+  /**
+   * Retrieve a source by id
+   *
+   * @param sourceId the source's id
+   * @return the source if present in the current style
+   */
   @Nullable
   @UiThread
   public Source getSource(@NonNull String sourceId) {

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

@@ -782,6 +782,13 @@ final class NativeMapView {
     nativeRemoveLayer(layer.getNativePtr());
   }
 
+  public List<Source> getSources() {
+    if (isDestroyedOn("getSources")) {
+      return null;
+    }
+    return Arrays.asList(nativeGetSources());
+  }
+
   public Source getSource(@NonNull String sourceId) {
     if (isDestroyedOn("getSource")) {
       return null;
@@ -1071,6 +1078,8 @@ final class NativeMapView {
 
   private native void nativeRemoveLayer(long layerId);
 
+  private native Source[] nativeGetSources();
+
   private native Source nativeGetSource(String sourceId);
 
   private native void nativeAddSource(long nativeSourcePtr) throws CannotAddSourceException;