Fixes concurrent display test flakiness

Registers a DisplayManager listener if the
rear display is not yet available when we
get into the concurrent display state. This could
occur if we are notified that we are now in the
concurrent display state while DisplayManager is still
in the process of configuring itself for this state.

Fixes: 267563768
Test: ExtensionRearDisplayPresentationTest
Change-Id: Ice4c111491f4096195518dfc38bee2f07d2c183f