Fix a11y in brightness slider

Put the clickable (when restricted) inside a `thenIf`. That way, it
won't mark the whole slider as disable when just the click behavior is
disabled.

Also, add the content description for the brightness slider.

Test: manual, talkback, set-user-restriction
Fixes: 412320165
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Change-Id: I3d660b8507600e87f4d3e2b9c1cbb06026bee7f8