UI test automation cannot get the root node and gets null children.

1. The AccessibilityInteractionController was using an incorrect
   looper i.e. not the UI thread looper which was causing getting
   the root node to fail.

2. The AccessibilityNodeInfo was populated by a ViewGroup with the
   children for accessibility without checking whether these children
   are really displayed.

bug:6362875

Change-Id: I7906d89571eb9d57d10f971639f88632926dd077