Fix Storage Settings summary jank

When switching dark theme, Settings homepage is re-created
and storage summary will be calculated in worker thread and
then be displayed after a short time.

Use cache to display the summary before latest summary is available.

Bug: 255459774
Test: manual visual
      Switch Dark theme and observe Storage settings summary.
Change-Id: I88c7190758a8bdf3a1a24137b9638ca3fb839887