Accessibility focus traversal in virtual nodes.

1. Finished the implementation of support for maintaining
   accessibility focus in view with virtual descendants.

2. Finished the NumberPicker implementation of virtual
   subtree such that all requred attributes are reported
   and ensuring that it support accessibility focus in
   its virtual descentants.

3. Fixed a bug where if a predecessor of the view that is
   accessiiblity focused is removed the accessibliity focus
   host in ViewRootImpl is not cleared leading to a crash
   when trying to draw the accessibility focus highlight.:

bug:6472646
bug:6433864

Change-Id: I3645642b87b4a26025c0b2ba9dfaad92d11a48f1