Refactor SummaryLoader to set summary on subsetting pages.

- Make a SummaryConsumer interface. Things that needs latest summary
  should implement this interface (DashboardAdapter for homepage,
  DashboardFragment for subsettings). This also decouples SummaryLoader
  from relying on SettingsDrawerActivity.
- Make DashboardFeatureProvider more generic to load DashboardCategory
  by key.

Bug: 31781480
Test: RunSettingsRoboTests
Change-Id: I9c65456fb433a74c352498251e0ccf65da0be1f0