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
Merge "storage: Don't list removed volumes in storage list" into cm-11.0