Show "Files" category for public storage in Storage Settings

Users can click this preference to see public storage files
in external file browsers.

Bug: 170918505
Test: make RunSettingsRoboTests ROBOTEST_FILTER=StorageItemPreferenceControllerTest
      manual
      Select private storage and public storage and observe UI
Change-Id: If3f0d59b2ebf941d70e81e60c4c2015b80e7cf33