Scale Animation Bug fix

The ZoomPanLayout.smoothScaleTo() wasn't animating, only moving.  
These changes should address that.

Change details:
-Changed System.currentTimeMillis() to SystemClock.elapsedRealtime() for
better animation reliability around clock changes. (This is not necessary and you can ignore if you prefer the previous way)
-Added call to ZoomPanLayout.saveHistoricalScale() in TweenListener this should fix a bug that was preventing scale based animations