Make mapElementsLazily thread-safe

in the previous version onEach could have been executed concurrently by several collectors. This new version doesn't have any mutable state.

Also making enabledDisplay an hot StateFlow.

Bug: 345472038
Test: DisplayRepositoryTest
Flag: com.android.systemui.enable_efficient_display_repository
Change-Id: Ifb3466f7c2a3216d6f9d7bed28a99d3edfb4d90f