[flexiglass] Use a key for every value saved by rememberSession

Not providing a key to the saved value results in using an automatically
generated key by the Compose runtime which is changing every time the
Composable "moves" in the composition tree e.g.
NotificationScrollingStack moving to QuickSettingsScene.

Bug: 405083492
Test: go from Single Shade to QS -> saved values are preserved
Flag: com.android.systemui.scene_container
Change-Id: I29e846d4d389d8b471c2beb84a3ba350c8de1036