Cleanup overview threshold which was only needed for 2-button mode

- Removed hasReachedOverviewThreshold() and mPassedOverviewThreshold
- Changed calculateEndTargetForNonFling() to check velocity.y to
  determine whether to go HOME or LAST_TASK when no other conditions are
  met (e.g. mIsMotionPaused still goes to RECENTS). To allow transient
  taskbar to override this so that user stays in LAST_TASK when invoking
  the taskbar, kept setHasReachedHomeOverviewThreshold() but renamed it
  to setCanSlowSwipeGoHome(),

Test: TaplTestsQuickstep, manually tested on tablet and phone to ensure
the correct end target was chosen when hitting this code path in
calculateEndTargetForNonFling
Flag: teamfood (ENABLE_TRANSIENT_TASKBAR)
Fixes: 259715295

Change-Id: I925be3433d7d2792cdc2a75a18dc575019f2cc95