Fix finding views by accessibility id.

1. If a view has a node provider, then the latter takes over
   representation of the view tree. Hence, find by accessibility
   id should not return children of a view with a provider.

2. Views can change their importantce for accessibility, so an
   accessibility service may get a node and try to act on it
   but the node became not important in the meantime. Hence,
   find by accessibility id should respect importance.

Change-Id: Ib4d738c00f46c91300605a2928550f40705ea47b