base

Exploring transitions to/from Recents.

- refactored hwlayers and change view property animations to use a reference counted trigger
- cleaned up RecentsConfiguration, and move it into classes using it
- moved task bar animations back into TaskBarView
- refactoring enter/exit animations to use an animation context

Change-Id: Ia66b622b094f22145c2fab07c2a9bdfd62344be2