[a11y] Change a11y semantic tree FIDL to use KOID

This changes the API and mostly ports existing code to work with KOIDs. Hit test logic will be deleted in a later CL.

TEST: build/run/manual testing, fx run-test a11y_tests
MI4-1736: #comment Changes to semantic tree FIDL

Change-Id: I8810d1ccb89b2dbc1debc422d7553cba83f02aaf
9 files changed