Preferences: Add autocomplete="off" to preferences form

Preferences: Add autocomplete="off" to preferences form

This change adds a new method setAutocomplete to the class HTMLForm.
This method allows to set the HTML attribute autocomplete for the form.
This change uses this method to set autocomplete="off" for the preferences form.

Without autocomplete="off" the selections in the preferences get cached in
the browser. This can lead to wrong selected options when the settings get
changed on an other way, for example via API.

Bug: T131047
Change-Id: I2920383b5b8cfca3f1d546315f202985edf417d8