Fix race condition and optimize categoryUpdater refresh

- In SettingsActivity, do not call updateCategories() if nothing
  changed.
- In SummaryLoader, create a mapping between tile key and summary. This
  is necessary to handle a race condition where category is refreshed
  after summary load.
- In DashboardSummary, refresh Tile's summary to latest cache value
  everytime category is refreshed.

Change-Id: I61389b8ba614ba7e34939325bada6e1bd6fa6709
Fix: 63149109
Test: robotests