Prevent HTML Injection on the Device Admin request screen

The root issue is that CharSequence is an interface.
String implements that interface, however, Spanned class
too which is a rich text format that can store HTML code.

The solution is enforce to use String type which won't include
any HTML function.

Test: Rebuilt apk and see the string without HTML style.
Bug: 179042963
Change-Id: I53b460b12da918e022d2f2934f114d205dbaadb0