Use appBounds when computing split screen aspect ratio

When computing the split screen aspect ratio we were previously using the bounds of the display which did not take insets into account, so an activity that would be using this value would get letterboxed when in split screen because the aspect ratio restriction is applied for appBounds on the activity.

Fix: 273513446
Test: atest WmTests:SizeCompatTests#testOverrideSplitScreenAspectRatio_splitScreenActivityInPortrait_notLetterboxed
Test: atest WmTests:SizeCompatTests#testOverrideSplitScreenAspectRatio_splitScreenActivityInLandscape_notLetterboxed
Change-Id: I996651124c16e855910efa3b315ea644adffd7c1