[flexiglass] Use actual media host locations

- Remove initial size setup in MediaCarousel (width would always be 0 there)
- Do update location in  MediaHierarchyManager so we calculate a new
  size when moving to a different host
- Remove minimum size workaround as it is no longer needed

Fixes: 340215986
Flag: com.android.systemui.scene_container
Test: manual - play media from multiple apps, verify carousel and
controls work as expected

Change-Id: Ic846d4b2bb7c09fa465ce76a2a1aecf9f49eac98