Per-app media directories on external storage.

This change defines per-app directories on external storage that
will be scanned and included in MediaStore.  This gives apps a way
to write content to secondary shared storage in a way that can
easily be surfaced to other apps.

Bug: 14382377
Change-Id: I6f03d8076a9391d8b9eb8421ec3fc93669b3ba0d