Fix timestamps on android touch events to use milliseconds, to be consistent with iOS

Summary:
So `PanReponder.onPanResponderRelease/onPanResponderTerminate` receive a `gestureState` object containing a `onPanResponderTerminate.vx/vy` property. On Android and iOS, they appear to be orders of magnitude different, which appear to be due to the different scale of timestamps that are used when generating touch events.

This pull request fixes the timestamps to be milliseconds on both platforms (since I assume iOS is the more authoritative one, and is the one that `react-native-viewpager`'s vx thresholds written written to compare against.)

As far as I can tell, the RN code doesn't use the `vx/vy` properties, so they should be okay. And looks like the RN code only cares about relative values of `startTimestamp/currentTimestamp/previousTimestamp` though, so should be fine too. it's quite possible there will be downstream android breakage with this change, particularly for those who are already compensating for the RN discrepancy.
Closes facebook/react-native#8199

Differential Revision: D3528215

Pulled By: dmmiller

fbshipit-source-id: cbd25bb7e7bb87fa77b661a057643a6ea97bc3f1

Author: james-watkin

ReactViewPager.java

@@ -91,14 +91,14 @@ private class PageChangeListener implements OnPageChangeListener {
     @Override
     public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
       mEventDispatcher.dispatchEvent(
-          new PageScrollEvent(getId(), SystemClock.nanoTime(), position, positionOffset));
+          new PageScrollEvent(getId(), SystemClock.elapsedRealtime(), position, positionOffset));
     }
 
     @Override
     public void onPageSelected(int position) {
       if (!mIsCurrentItemFromJs) {
         mEventDispatcher.dispatchEvent(
-            new PageSelectedEvent(getId(), SystemClock.nanoTime(), position));
+            new PageSelectedEvent(getId(), SystemClock.elapsedRealtime(), position));
       }
     }
 
@@ -119,7 +119,7 @@ public void onPageScrollStateChanged(int state) {
           throw new IllegalStateException("Unsupported pageScrollState");
       }
       mEventDispatcher.dispatchEvent(
-        new PageScrollStateChangedEvent(getId(), SystemClock.nanoTime(), pageScrollState));
+        new PageScrollStateChangedEvent(getId(), SystemClock.elapsedRealtime(), pageScrollState));
     }
   }