Bug 830239 - When scanning SD Card first time, communicate this to the user. r=dflanagan, l10n

Fix for [#830239](https://bugzilla.mozilla.org/show_bug.cgi?id=830239). When you start up your phone with an empty media database the phone tells you that there are no photo's yet. This is untrue. This adds another status message that tells the user that the SD card is being scanned when that is happening.

Redo'ing #8450.