Launch StorageDashboardFragment instead of StorageSettings

Since users can use StorageDashboardFragment to choose storages,
it's not necessary to choose storages in StorageSettings.

Bug: 174964885
Test: make RunSettingsRoboTests ROBOTEST_FILTER=LowStorageSliceTest
      manual
      Launch Storage Settings.
Change-Id: Ida61052c4294899099914d62da24ca2937a894a0