Fix new back arrow's "over eager" appearance

There are two fixes in this cl:
1. The back arrow launches into its ENTRY state too easily: guard it
   with a minimum motion threshold.
2. The cancel animation is triggered upon MotionEvent.ACITON_UP, even
   when there has not been sufficient motion to trigger a change from
   GONE -> ENTRY (invisible to appearance): prevent GONE -> CANCELLED
   state transitions.

Bug: 238907408
Test: manual
Change-Id: I5ab8042fc675cfc527b837578b1d415d2d061d79