Add option to track when we're showing blankness during fast scrolling

Summary:
If tracking is enabled and the sampling check passes on a scroll or layout event,
we compare the scroll offset to the layout of the rendered items. If the items don't cover
the visible area of the list, we fire an `onFillRateExceeded` call with relevant stats for
logging the event through an analytics pipeline.

The measurement methodology is a little jank because everything is async, but it seems directionally
useful for getting ballpark numbers, catching regressions, and tracking improvements.

Benchmark testing shows a ~2014 MotoX starts hitting the fill rate limit at about 2500 px / sec,
which is pretty fast scrolling.

This also reworks our frame rate stuff so we can use a shared `SceneTracking` thing and track blankness
globally.

Reviewed By: bvaughn

Differential Revision: D4806867

fbshipit-source-id: 119bf177463c8c3aa51fa13d1a9d03b1a96042aa

Libraries/Utilities/__tests__/SceneTracker-test.js

@@ -0,0 +1,30 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+'use strict';
+
+jest.unmock('SceneTracker');
+
+const SceneTracker = require('SceneTracker');
+
+describe('setActiveScene', function() {
+
+  it('can handle multiple listeners and unsubscribe', function() {
+    const listeners = [jest.fn(), jest.fn(), jest.fn()];
+    const subscriptions = listeners.map(
+      (listener) => SceneTracker.addActiveSceneChangedListener(listener)
+    );
+    subscriptions[1].remove();
+    const newScene = {name: 'scene1'};
+    SceneTracker.setActiveScene(newScene);
+    expect(listeners[0]).toBeCalledWith(newScene);
+    expect(listeners[1]).not.toBeCalled();
+    expect(listeners[2]).toBeCalledWith(newScene);
+  });
+});