Call initTransitionEndpoints in constructor

Before, the soonest we initTransitionEndpoints() is in
onRecentsAnimationStart(), which might be too late if launcher was
killed. (In that case, the gesture might end before we ever get
onRecentsAnimationStart(), and thus will never consider the distance
traveled great enough to trigger recents since we don't have endpoints.)

Bug: 129723135
Change-Id: I349f62244aaba8369926b14f90acd994fd40a93a