Settings: add key to USB Storage Unmount option

If we want to be able to highlight this preference and we need a key to do so.

Change-Id: I8c78f8cf8e75bbc071438ef4f733f3ac49e27791
Signed-off-by: Roman Birg <roman@cyngn.com>
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0