Update volume panel in-call behaviour:

1) Show call slider on top;
2) Move isExpanded updates to be behind the `StateFlow`. This fixes the case when the media session is updated, the playback doesn't change and mutableIsExpanded is overridden by that update;

Flag: EXEMPT bugfix
Fixes: 353589592
Fixes: 346891818
Test: manual on foldable
Test: atest VolumePanelScreenshotTest
Change-Id: Id0591da61484fb416b6eda4aaf74c49e447cf0aa