Remove quick switch and simplify quickstep code for scrub

Remove quick switch code and the concept of a threshold for starting
quickscrub from system ui. Quickscrub starts (and is sent to launcher)
after touch slop (in the direction parallel to nav bar) is passed.

Change-Id: I59fa25c9a9d50298adcf877590b3beee6d887308
Fixes: 73820325
Test: drag home button when quick step enabled