add directional accessibility page actions to StackView

StackView has ITEMS_SLIDE_UP mode and ITEMS_SLIDE_DOWN mode. In
ITEMS_SLIDE_UP mode, scroll forward is slide up and all pages go up
(page down action), scroll backward is slide down and all pages go down
(page up action). In ITEMS_SLIDE_DOWN mode, it is the opposite.
A video in ITEMS_SLIDE_UP MODE is here:
https://drive.google.com/file/d/1m5awAP7ja8tqSaSvNM_oDoVbaUeCa3l6/view?usp=sharing

Test: Tested with twisted talkback that in ITEMS_SLIDE_DOWN mode, page
down action is equivalent to scroll backward, and page up is equivalent
to scroll forward.
Bug: 136277517

Change-Id: I52b2af231781fa04cec5815db2e573a9014e434b