Interrogation of the view hierarchy from an AccessibilityService.

1. Views are represented as AccessibilityNodeInfos to AccessibilityServices.

2. An accessibility service receives AccessibilityEvents and can ask
   for its source and gets an AccessibilityNodeInfo which can be used
   to get its parent and children infos and so on.

3. AccessibilityNodeInfo contains some attributes and actions that
   can be performed on the source.

4. AccessibilityService can request the system to preform an action
   on the source of an AccessibilityNodeInfo.

5. ViewAncestor provides an interaction connection to the
   AccessibiltyManagerService and an accessibility service uses
   its connection to the latter to interact with screen content.

6. AccessibilityService can interact ONLY with the focused window
   and all calls are routed through the AccessibilityManagerService
   which imposes security.

7. Hidden APIs on AccessibilityService can find AccessibilityNodeInfos
   based on some criteria. These API go through the AccessibilityManagerServcie
   for security check.

8. Some actions are hidden and are exposes only to eng builds for UI testing.

Change-Id: Ie34fa4219f350eb3f4f6f9f45b24f709bd98783c