Settings: Fix cut off brightness slider

The brightness slider gets cut off in multi-window and freeform window
because the top margin is too short. This change adds a padding for top
margin when in multi-window or freeform.

Bug: 389946114
Test: Manually test brightness slider in multi-window and freeform modes
Flag: EXEMPT bugfix
Change-Id: Id10c1ade9efe32f06bba3e33bec314f62009302b