[flexiglass] destinationScenes a stable flow.

Changes destinationScenes from fun to val but, more importantly, changes
all implementations to return a constant value for the flow. For
implementations that return a more complex Flow, this doesn't change
anything but for those that return a newly constructed MutableStateFlow,
it prevents each property read from instantiating a new MutableStateFlow
copy.

Fix: 300915579
Test: moved between scenes in Flexiglass, visited all scenes except
Communal

Change-Id: I9ef5a7d9fc1c665909e3e5e600e18057d0058887