Fix race condition in screenshot code with multiple displays

DisplayRepository.displays flow was initialized as an empty set at first when converted to a StateFlow.
Now it is a SharedFlow without any initial values: consumers of the flow will have to wait to get the first value.
As the replay number is equal to 1 and onStart emits an event, everyone collecting the flow will have something immediately and not starve.

Fixes: 306680094
Test: TakeScreenshotExecutorTest, DisplayRepositoryTest
Change-Id: I6ac7f3d6aa31f7f3fff4a5691a51bf8db33021df