Improved the behavior of overscroll

The overscroll logic could get stuck previously, which lead to
a forehead visible on the lock screen.
This improves the logic and animation to look much better.

Fixes: 190029393
Test: expand shade, observe nice overshoot, no forehead on lockscreen
Change-Id: I2c9d5313626d3037c7bd3c942820c391da99b459