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