[flexiglass] Scroll the shade with TalkBack

Populate accessibility nodes with the current scroll state, to restore
the pre-flexiglass functionality of navigating through notifications
by just swiping from left to right (with TalkBack). Before this change
TalkBack stops at the last visible notification and doesn't scroll down
to the next, if the shade has enough notifications to scroll.

Bug: 363260043
Test: use TalkBack to navigate through notifications and footer buttons in the shade
Flag: com.android.systemui.scene_container

Change-Id: I38f07e75afdb988f5aca4c81930a2dc5c7915759