do not modify frozen string when bulding device overview

Bnc906694
and you do try it with different LANG? Because in english ENV it return string itself, which is not frozen.
But I try it manually and looks good:

```
irb(main):004:0> t = "test"
=> "test"
irb(main):005:0> t.freeze
=> "test"
irb(main):007:0> t.frozen?
=> true
require "yast"
=> true
Yast.import "HTML"
=> nil
res = Yast::HTML.Colorize(t, "red")
=> "<font color=\"red\">test</font>"
irb(main):012:0> res.frozen?
=> false
```
```