Add back a check for node provider when determining accessibility importance

ag/23011915 removed this check, but this removal means views that
override getAccessibilityNodeProvider may not be included for
accessibility.

Fixes: 323027946
Bug: 323027946
Test: manual with Slides, cts test
Change-Id: I91f72c8d60bda813c8d2740f8a16fb73b2d5f8d8