Prepare ExpandableNotificationRow for new A11y expansion APIs

Expose the row's current expanded state to accessibility services by
using AccessibilityNodeInfo#setExpandedState.
When the expandable state changes, dispatch a new accessibility event.

Bug: 380027122
Test: expand/collapse Notifications with TalkBack
Flag: com.android.systemui.notification_row_accessibility_expanded
Change-Id: Iff1b198daab597f3427e439c8031b8243a5bf42c