Refine simulateLayoutDisplay

Instead of calling into layoutWindowLw while simulateLayoutDisplay, this
CL invokes WindowLayout#computeWindowFrames directly. All windows will
compute their frame locally, so we don't need layoutWindowLw then.

Bug: 161810301
Test: atest DisplayPolicyLayoutTests SizeCompatTests
Change-Id: If123f4d56758078def8a7c50c218d75e0217633b