Adjust smallestScreenWidthDp for letterbox

By definition, smallestScreenWidthDp is the smallest value of both screenWidthDp and screenHeightDp. When a different value is reported in smallestScreenWidthDp, the app can load incorrect layout.

Test: atest WmTests:SizeCompatTests
Fix: 232406936
Change-Id: Ie0d7767fba03cc07e21e3624d9d8c5b0ea13dff1