Make sure _everything_ is unmounted in MultiROM::restoreMounts()

Change-Id: Idf7d7f71a0a1c97a9f91c0c95205a94017144615
Fix fileselector showing empty screen when dir's content changes

* When fileselector is scrolled to the bottom of folder with
  a lot of files and then the folder is reloaded and it no longer
  has as much files, it can stay scrolled "past" the current
  file list and display only a blank space even though the folder
  has files.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>

Change-Id: Ic8f5e1d85c7c177f7abb07b3f1b34f20111c4eb4