WinScope: Support boundless surfaces in WinScope

SurfaceFlinger trace relies on Display Root#0 layer size to set the display bounds. This layer will
no longer have a size once b/114413815 is fixed. Instead, sf transform will set the bounds to undefined and the
bounds be calculated to be the union of the layer sizes.
see: http://x20web/teams/android-window-manager/winscope/winscope-staging-vishnun.html
Bug: 114413815
Test: build winscope and open trace

Change-Id: Ic04d524f0e095374321ef818f0d0ff420a2d3827