Improving drag and drop animations.

- Expanding drop targets to indicate the size of the to-be docked window
- Fixing animation when dropping task
- Fixing drag view z order
- Fixes issue where the dock divider position in WM is not exact
- Requiring user to move the slop distance before accepting drops

Change-Id: I2f6eab504db7126c19e0c680629e89a39e7512e3