DisplayTopology: make allNodesIdMap public

This is a more light-weight utility than getAbsoluteBounds for getting
all display IDs in the topology. It is also needed by ag/32951858 to
resolve a display ID to its logical size.

Flag: EXEMPT refactor
Bug: b/397231553
Test: SQ
Change-Id: I2a1baed6c84f4078f18860b3a291dae76d3ea652