Make topology validator a static function

This CL removes the isValid method from DisplayTopologyGraph and adds it
as a static method in DisplayTopologyGraph.

This is a follow up from ag/32211998.

Test: atest DisplayTopologyGraphTest/DisplayTopologyGraphTestFixture
Bug: 401219231
Flag: com.android.input.flags.enable_display_topology_validation
Change-Id: Ib853c5b39ce956737bb547917043da29492ff97b