Hold StateFlow for focusedDisplayId in ScreenshotProxyService

This CL changes ScreenshotProxyService to hold the StateFlow for
focusedDisplayId rather than accessing it on-demand.
As FocusedDisplayRepository#focusedDisplayId is not a StateFlow but its
getter generates new StateFlow from the original cold flow, it’s not
supposed to be inlined, which results in returning the default value
before the cold flow emits the correct value.

Bug: 399644508
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Flag: com.android.systemui.screenshot_multidisplay_focus_change
Test: manual - connect an external display and screenshot
Change-Id: Id72c4ae2cc673980915ac77aff3c3aea97956367