storage: Don't list removed volumes in storage list

 * Don't tell apps that USB storage or empty SD slots are available for
   use. This fixes errors about trying to create dirs in vold as well
   as the relevant CTS cases.

Change-Id: Ic763d40c19b0bb9f7497cc1515baf1661d76e9ee
(cherry picked from commit 5a3c6b05)