Remove max displacement threshold in 2-zone model

We had a max displacement for detecting pause before because people
were falsing into overview when they dragged all apps all the way
up. In this mode, there's no way to get to all apps from the nav
bar so the threshold doesn't make sense.

Bug: 151039912
Change-Id: I349f38ec1589f8b151cfbe32542159b3eb92bf61