Improving accessibility APIs used for UI automation.

1. UiTestAutomationBridge was accessing the root node in the
   active window by tracking the accessibility event stream
   and keeping the last active window changing event. Now
   the bridge is stateless and the root node is fetched by
   passing special window and view id with the request to
   the system.

2. AccessibilityNodeInfos that are cached were not finished,
   i.e. not sealed, causing exception when trying to access
   their children or rpedecessors.

3. AccessibilityManagerService was not properly restoring its
   state after the UI automation bridge disconnects from it.
   I particular the devices was still in explore by touch mode
   event if no services are enabled and the sutomation bridge
   is disconnected.

4. ViewRootImpl for the focused window now fires accessibility
   events when accessibility is enabled to allow accessibility
   services to determine the current user location.

5. Several missing null checks in ViewRootImpl are fixed since
   there were scenraios in which a NPE can occur.

6. Update the internal window content querying tests.

7. ViewRootImpl was firing one extra focus event.
bug:6009813
bug:6026952

Change-Id: Ib2e058d64538ecc268f9ef7a8f36ead047868a05