Only create custom database paths.

If the getDatabasePath() caller is using the default getDatabasesDir()
path, that method already handles possible path creation, so no
need for us to retry.

Also fix two small bugs that would have NPE'ed when listing files in
a directory that wasn't created.

Bug: 26895777
Change-Id: Iced16b05cfe95062717da8d0414c48daefcb5648