Sync scroll position between glanceable hub and edit mode.

This changelist saves the latest scroll position when entering and
exiting edit mode so the grid is at the same position between both
views.

Test: atest CommunalViewModelTest#scrollPosition_persistedOnEditEntry
Test: atest CommunalEditModeViewModelTest#scrollPosition_persistedOnEditCleanup
Fixes: 343463302
Flag: com.android.systemui.communal_hub
Change-Id: Ibf4f8de4fc125bf1979e1cc0293681e338102e24