Initial implementation of requestIdleCallback on Android

Summary:
This is a follow up of the work by brentvatne in #5052. This addresses the feedback by astreet.

- Uses ReactChoreographer with a new callback type
- Callback dispatch logic moved to JS
- Only calls into JS when needed, when there are pending callbacks, it even removes the Choreographer listener when no JS context listen for idle events.

** Test plan **
Tested by running a background task that burns all remaining idle time (see new UIExplorer example) and made sure that UI and JS fps stayed near 60 on a real device (Nexus 6) with dev mode disabled. Also tried adding a JS driven animation and it stayed smooth.

Tested that native only calls into JS when there are pending idle callbacks.

Also tested that timers are executed before idle callback.
```
requestIdleCallback(() => console.log(1));
setTimeout(() => console.log(2), 100);
burnCPU(1000);
// 2
// 1
```

I did *not* test with webworkers but it should work as I'm using executor tokens.
Closes https://github.com/facebook/react-native/pull/8569

Differential Revision: D3558869

Pulled By: astreet

fbshipit-source-id: 61fa82eb26001d2b8c2ea69c35bf3eb5ce5454ba

Libraries/JavaScriptAppEngine/System/JSTimers/JSTimers.js

@@ -96,6 +96,41 @@ var JSTimers = {
     return newID;
   },
 
+  /**
+   * @param {function} func Callback to be invoked every frame and provided
+   * with time remaining in frame.
+   */
+  requestIdleCallback: function(func) {
+    if (!RCTTiming.setSendIdleEvents) {
+      console.warn('requestIdleCallback is not currently supported on this platform');
+      return requestAnimationFrame(func);
+    }
+
+    if (JSTimersExecution.requestIdleCallbacks.length === 0) {
+      RCTTiming.setSendIdleEvents(true);
+    }
+
+    var newID = JSTimersExecution.GUID++;
+    var freeIndex = JSTimers._getFreeIndex();
+    JSTimersExecution.timerIDs[freeIndex] = newID;
+    JSTimersExecution.callbacks[freeIndex] = func;
+    JSTimersExecution.types[freeIndex] = JSTimersExecution.Type.requestIdleCallback;
+    JSTimersExecution.requestIdleCallbacks.push(newID);
+    return newID;
+  },
+
+  cancelIdleCallback: function(timerID) {
+    JSTimers._clearTimerID(timerID);
+    var index = JSTimersExecution.requestIdleCallbacks.indexOf(timerID);
+    if (index !== -1) {
+      JSTimersExecution.requestIdleCallbacks.splice(index, 1);
+    }
+
+    if (JSTimersExecution.requestIdleCallbacks.length === 0) {
+      RCTTiming.setSendIdleEvents(false);
+    }
+  },
+
   clearTimeout: function(timerID) {
     JSTimers._clearTimerID(timerID);
   },
@@ -127,7 +162,9 @@ var JSTimers = {
     // See corresponding comment in `callTimers` for reasoning behind this
     if (index !== -1) {
       JSTimersExecution._clearIndex(index);
-      if (JSTimersExecution.types[index] !== JSTimersExecution.Type.setImmediate) {
+      var type = JSTimersExecution.types[index];
+      if (type !== JSTimersExecution.Type.setImmediate &&
+          type !== JSTimersExecution.Type.requestIdleCallback) {
         RCTTiming.deleteTimer(timerID);
       }
     }