Implement transition for docking task in recents #6

- Use a future to provide the app thumbnail so the app can restart
in parallel when recents draws the bitmap (extremely expensive).
- Don't call startRecents from AM when recents is already running - this
messes up the transition information.
- Make sure to put the task into resizing mode if it needs to be restored
from the disk.
- Some minor fixes for the transition animation spec.
- Add NO_MOVE_ANIMATION to recents flags to prevent wallpaper
flickering.

Bug: 27607141
Change-Id: I7d0c75b88775ab467927b8cf94303ddb60222e7f