API for finding accessibility focus in virtual tree not needed.

1. The function for finding where the accessibility focus in a virtual
   node tree presented by an AccessibilityNodeProvider is not needed
   API since the framework already keeps track of the accessibility
   focused virtual node in order to draw the focus rectangle. This API
   adds unnecessary complexity to developers of AccessibilityNodeProviders.

bug:6675330

Change-Id: I84774686b06a995073a39e45b8ef22f2cd04b773