Add task snapshot for home task

The real home activity usually takes longer time than a
snapshot to complete drawing. Especially when unlocking
to home, the duration becomes more noticeable.

This CL enables snapshot of home task when turning screen
off. And because the snapshot is only used when entering
home from a sleeping state, the snapshot will be removed
once it is used.

Also provide a method for home activity to remove its
snapshot while the content of home activity has significant
change after taking the snapshot.

Bug: 140811348
Test: 1. Set an unlock method that doesn't need to show
         lockscreen when unlocking.
      2. Turn off screen while home is on top.
      3. Unlock device to enter home.
      4. Check the duration of trace "screenTurningOn".

Change-Id: Ic516cdcfd84ca4a85e19798537f9f5a85077392b