Fix failing DesktopDisplayEventHandlerTest

Replaces the DesktopRepositoryInitializer mock with a fake. Stubbing a
property/val with mockito fails sometimes and results in an NPE.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 401298572
Test: atest WMShellUnitTests (only reproduced when running the entire
module)

Change-Id: I7205ee8a602927fc7aa1cac3d641978cf25140df