Add the ability to browse all the device storages available on the device

Starting with FxOs 1.1, several device storages can be used on a device.
For example, the Peak device has its internal storage and  the sd-card

With current version, it only displays the content of the default device storage (chosen in the settings).
This patch adds the ability to browse all the device storages available.
If there is more than one, a drop/down list appears at the top of the screen, which lets the user select the device storage to browse

(this also includes a small fix on prefix stripping when the name of the current device storage is not "sdcard")

PS : I don't know why github considers all the content of files to be modified : I only modified/added a few lines