Ignore the negative minWidth/minHeight populated from windowLayout

When an activity specifies defaultWidth/defaultHeight in <layout>
without minWidth/minHeight, -1 will be populated into windowLayout for
those fields.

Bug: 152933995
Test: manual enter/exit PiP from Duo app
Change-Id: I9056891f2baafbad9268a45a6820ec515d6f1b84