BackMotionEvent: add velocity (in pixels/second)

The velocity of the `BackMotionEvent` will be used to fire additional events when the user lifts their finger.

Note: Velocity is calculated for the last event only, for performance reasons (see `VelocityTracker.computeCurrentVelocity`).
`Float.NaN` will indicate that the value has not been calculated.

Test: atest BackAnimationControllerTest
Bug: 263402927
Change-Id: I301636f572eb4e59abc69f6fee11d8dba5647a16