Fix back gesture send to wrong focus window

Ensure we could get the proper focused window token what it's during
updating the focus if user touched down on another non-focused window.

It also called `startBackNavigation` when received first move that next
to down event to ensure the `WMS.onPointerDownOutsideFocus` could be
processed first.

Test: atest BackAnimationControllerTest BackNavigationControllerTests
Test: enabled split-screen, trigger back gesture on non-focused window
Bug: 230442550
Bug: 227724016
Change-Id: I81c4f02d43299c03f94dc295bb23ba80d46bded8
Merged-In: I81c4f02d43299c03f94dc295bb23ba80d46bded8