livedisplay: Float-to-string conversions considered harmful

 * Force US locale when doing internal flattening of HSIC objects
   into strings. This would crash in the validator if the user's
   local uses commands instead of dots to format floats.

RM-290

Change-Id: I08f44238a486308a483205b97632114d2f7a77f1
(cherry picked from commit db640a96)