Make entering recents a lot faster

- Precache the bitmap for the window animation in the preload phase
- Remove some post's so we have a faster path from UP -> startActivity
- Don't dim the headers in the first frame drawn, because layer
creation is slow. Instead, do it in the second frame, when the window
animation is already running.

All these changes combined make going to recents about 40-50ms faster.

Change-Id: I3e4060af1ac57b3f359fe7f86f9e3814c6490323