Remove exported constants from RCTTimers to allow lazy initialization

Summary:
As per javache comments in #8734.

Also removes now useless feature detection check.

**Test plan**
Tested that rIC still works in UIExplorer example.
Closes https://github.com/facebook/react-native/pull/8795

Differential Revision: D3572566

Pulled By: javache

fbshipit-source-id: 261d13d8b03898313f8b4184d634c70f81a61b62

ReactAndroid/src/main/java/com/facebook/react/modules/core/Timing.java

@@ -45,6 +45,7 @@ import javax.annotation.Nullable;
 public final class Timing extends ReactContextBaseJavaModule implements LifecycleEventListener,
   OnExecutorUnregisteredListener {
 
+  // These timing contants should be kept in sync with the ones in `JSTimersExecution.js`.
   // The minimum time in milliseconds left in the frame to call idle callbacks.
   private static final float IDLE_CALLBACK_FRAME_DEADLINE_MS = 1.f;
   // The total duration of a frame in milliseconds, this assumes that devices run at 60 fps.
@@ -309,13 +310,6 @@ public final class Timing extends ReactContextBaseJavaModule implements Lifecycl
     return true;
   }
 
-  @Override
-  public Map<String, Object> getConstants() {
-    return MapBuilder.<String, Object>of(
-        "frameDuration", FRAME_DURATION_MS,
-        "idleCallbackFrameDeadline", IDLE_CALLBACK_FRAME_DEADLINE_MS);
-  }
-
   @Override
   public void onExecutorDestroyed(ExecutorToken executorToken) {
     synchronized (mTimerGuard) {