Prevent out of range magnification scale to be persisted.

1. If a bad magnification scale is persisted, i.e. it is
   not between the min and max, the screen magnifier gets
   into a bad state which even a reboot does not fix since
   the scale is persisted in settings.

   This change ensures that only valid scales are presisted.
   In general a bad value should not be attempted to be
   persisted but at this point this is the safest change.

bug:7288239

Change-Id: I3e9c7c091772fa64128ab8403c2127ce65cb94b8