[DO NOT MERGE] Enforce user separation on external storage

With WRITE_MEDIA_STORAGE permission, MediaProvider can access external
storage across users.

Access should be rejected for content in other user's external storage
if the caller has no WRITE_MEDIA_STORAGE/INTERACT_ACROSS_USERS

Test: manual + android.provider.cts.MediaStore_FilesTest
Bug: 34114752
Change-Id: I577cc33574185657592246c1103df4174ff3acf6
(cherry picked from commit cc1cecc9be51f9655d9acdf1729345d16f078c21)