Polish for all apps physics.

By adjusting the start value based on the direction of the scroll,
the springs will appear more smooth.

This only changes the appearance of the spring when scrolling down,
since the start value has always been 1 and thus looked fine when
scrolling up.

Bug: 38349031
Change-Id: I563e6e7cfdbc74c4a95adb22f90d5efe17dfa453