FeatureFlag: quick scrub is now quick switch

- Scale down current task and translate it to the left
- Translate previous task in from the left
- This is a toggle; quick switch again returns to the first task

Currently this is implemented by repurposing onQuickScrub(progress) to update the positions
of the first two pages. This makes tracking velocity a bit difficult; if we want to go down
this path in the long run we should probably track velocity properly on MotionEvents.

Change-Id: I4445b5f08b6e88e71cbb5e30b1f1d45c5f1edc68