Issue #2: remove the dependency of RxJava

[whyalwaysmea]

I add method startDrop() and stopDrop() . It used Handler to start animation has frequency. I don't get ImageView in a new Thread, it's not normative. but it works normally. To be better, I think we can start a new Thread to get ImageView, then use Handler or post() start animation in mainThread. I'm not very good at View.