Fixed Brightness dialog UI dimensions

Brightness slider dimensions no longer takes the full width of the screen without any paddings. Added horizontal and vertical margins. Brightness slider will now take half width when in landscape mode and full width with paddings when in potrait.

Test: Trigger brightness dialog using keyboard shortcut, check that when in landscape mode the brightness dialog is on the left top corner of the screen and when in potrait mode its in the top center and takes full width except for the margins.
Fixes: 268460896
Change-Id: Iffb940c8af628c8184a82aaba7f931e01be3fa22