Expose getGraph() and DisplayTopologyGraph as TestApi

- Refactored AdjacentDisplay -> AdjacentEdge
- Modified displayId -> displayNode
- Refactored getGraph to build DisplayTopologyGraph in multiple steps
- Updated tests to create DisplayTopology and use getGraph() directly
instead of creating AdjacentDisplay manually

Bug: 412528980
Test: atest DisplayTopologyTest
Test: atest VisualIndicatorUpdateSchedulerTest
Test: atest DesktopMouseTestRuleTest
Flag: com.android.server.display.feature.flags.display_topology
Change-Id: I4913cdc7226836e92a79c81879b02f5b0a38f759