Virtual nodes are always important for accessibility.

1. Virtual nodes should be made important since the implementer of
   the tree represented by the nodes decides which node to report.
   In the case with native widgets we decide in the framework but
   in the case of the node provider, the implementer of the latter
   makes the call. Hence, if a node in not important the provider
   should not report it in the first place. The issue this patch
   solves is to allow events from virtual nodes to be propagated
   to the accessibility services.

bug:6432588

Change-Id: Ie01f84e9e0ef2280da934b98283962a5db38abc2