Cancel and dispose network requestHandles onDestroy (#1899)

* Cancel and dispose network requestHandles onDestroy

* Cleanup - addressing review comments

- removing accidental @keep
- use httpHandler directly

platforms/android/tangram/src/main/java/com/mapzen/tangram/MapController.java

@@ -275,6 +275,7 @@ void UIThreadInit(@NonNull final GLSurfaceView view, @Nullable final HttpHandler
      */
     protected void dispose() {
 
+        cancelAllNetworkRequests();
         httpHandler = null;
 
         // Prevent any other calls to native functions during dispose
@@ -1428,6 +1429,18 @@ public void onSurfaceCreated(final GL10 gl, final EGLConfig config) {
     // Networking methods
     // ==================
 
+    void cancelAllNetworkRequests() {
+        if (httpHandler == null) {
+            return;
+        }
+        synchronized (httpRequestHandles) {
+            for (int i = 0; i < httpRequestHandles.size(); i++) {
+                httpHandler.cancelRequest(httpRequestHandles.valueAt(i));
+            }
+            httpRequestHandles.clear();
+        }
+    }
+
     @Keep
     void cancelUrlRequest(final long requestHandle) {
         final HttpHandler handler = httpHandler;