Installer codex followup

Follow up I1f85110e754546c797d3325b8dd6878093790266:

• In WebInstallerDBConnect, fix incorrect "for" attributes on labels.
• Add the missing "for" attribute to checkbox labels.
• For radio buttons, append the codex class instead of replacing the class specified by the caller. This avoids breaking the showHideRadio events.
• For checkboxes, append the codex class even when a class is supplied by the caller. Checkboxes with classes were unclickable, because they lacked the codex class.

Bug: T337435
Change-Id: Id067002190a14d163f26270c668e7db3aca28b5d