Properly populates AccessibilityEvents for notification and quick settings shades

Previously, our method of announcing the expansion of the notification shade
and quick notification shade relied on populating a contentDescription
attribute on the highest-level container.  This behavior is incorrect as it
causes accessibility services to infer importance and assign the view
accessibility focus.  This change adds descriptive text to the
AccessibilityEvent fired when the shade opens without modifying the
container's description.

Bug: 8699624
Change-Id: Id909b60d63cdec96c307dea2624b0651ab4f1f36