feat(magnification fling): add fling momentum velocity tracking to panning gesture

Make the fullscreen magnification nice and slippery, like a wet dog ice
skating with skates made of butter.

Unfortunately, we can't add tests to
FullScreenMagnificationControllerTest, because it uses mock animators
and custom Handler timing, but Scroller is wired directly to the real
ones which we can't fake/mock.

Fix: 319175022
Flag: ACONFIG services.accessibility.fullscreen_fling_gesture DISABLED
Test: atest com.android.server.accessibility.magnification.FullScreenMagnificationGestureHandlerTest
NO_IFTTT=just TODO comments

Change-Id: Ic37278895846ed46604bb506ecf41177b94b0d17