Fix task bar unstashing and stashing when triggering back gesture using the trackpad

Need to defer starting recents animation. Instead of starting at touch down, will defer to passing initial slop. For the back gesture, launcher gesture nav will cancel the motion events from the input monitor passing the initial slop since it's not detected as a swipe up gesture.

Fixes: 276390218
Test: 3-finger horizontal swipe to go back, and make sure the task bar stay stashed through the transition
Change-Id: I03376c4d9696a9e5679b74e36ec142efa882981b