Modify focus traversal policy search to use focus tree instead of widget tree

[gspencergoog]

Description

When the focus tree and widget tree aren't the same shape, as when an explicit parentNode is supplied to a Focus widget, then the determination of the focus group is sometimes incorrect (Googlers, see b/270168643), because the algorithm uses an inherited widget lookup to find the nearest FocusTraversalGroup to determine the policy, which is incorrect.

This PR modifies the algorithm to traverse the focus tree instead, and introduces a static function FocusTraversalGroup.maybeOfNode , which takes a FocusNode and returns the policy in effect for it. It also modifies the of and maybeOf functions to use inherited widget lookup to find the nearest ancestor Focus or FocusScope widget, and then call maybeOfNode with their implicit FocusNode to determine the policy instead of searching for the FocusTraversalGroup ancestor directly, so that they'll follow the focus tree instead of the widget tree once they find a focus node.

To implement this, I also added a createDependency argument to Focus.of, Focus.maybeOf, and FocusScope.of, which allows callers to find a focus node in the widget tree without creating a rebuild dependency.

Tests

• Added a test to check that the correct policy is found when the widget and focus trees aren't the same shape.

[goderbauer]

LGTM