fix the issue which will broken when close the hardware acceleration in API19.

[mjkohoh]

In API19,if you set hardware acceleration in false.The SlidingLayer will throw a RuntimeException. The reasone is the DrawingCacheEnable has been set in false too early.

In my commit,I set DrawingCacheEnable in false only when mScrolling and mIsDraging is false.And make sure it is only be called when 'onDraw' is finished.I