Do not stateIn on every getter access

This CL prevents generating a new hot flow on every getter access. As
the signal gets updated in the bg thread, the caller had to hold the
flow in their class instead of inlining the access like
`FocusedDisplayRepository.focusedDisplayId.value`.

Bug: 399644508
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: manual - connect an external display and screenshot
Change-Id: I1cef3f9e44e6c93faf4c6bd054e935b093d4a79d