Default display topology initialization

Move the topology tree to a separate class for code readability.

The first external display is placed above the default display.

The subsequent displays are placed to the right of the rightmost display.

Bug: 364909693
Flag: com.android.server.display.feature.flags.display_topology
Test: DisplayManagerServiceTest, DisplayTopologyCoordinatorTest, DisplayTopologyTest, ExternalDisplayStatsServiceTest
Change-Id: I57012e52d327ade8dd8ecaebbbb9e197fb79c7d6