fix the issue when close hardware acceleration in API19.

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