Fix SysUI dialog width

This CL fixes the code that sets the dialog width such that:
 1. Dialogs don't match their parent width on phones in landscape.
 2. We take the background insets into consideration, so that the
    actual dialog size is exactly what we want it to be.

Before this CL, we wouldn't take the background insets into
consideration and therefore a 504dp width dialog was actually
504dp - 2 x 16dp = 472dp.

Bug: 203389579
Test: Manual
Change-Id: Ic66b043fe57ea7d493831e2b7d0e42a62362fd6f