Fix ViewPager crash for a11y z order.

The ViewPager requires measure/layout to initialize a list used
in getChildDrawingOrder, and crashes when this method is called
too early. Protecting against that by assigning a default value
to the drawing order value of AccessibilityNodeInfo if the View
bounds have not yet been set.

Bug: 27076826
Change-Id: Ic42dbb81d15340f6b021626b1c27ad5abce7e7d1