Add check for no children in VIew hosting virtual view tree.

1. For accessibility purposes View may report a virtual tree
   rooted at them to convey the logical structure they draw.
   These are usually custom Views. Such views cannot have
   children and this patch adds a check for that.

bug:6022217

Change-Id: I3795a613b4aef1a7ec5810db8584a11b648870f2