Use the display 0 transform for assertions

The first display is not always display 0. For example screen recording
creates a virtual display to duplicate the screen. The assertions should use data from display 0.

Bug: 188792659
Test: atest FlickerTests
Test: atest WMShellFlickerTests
Change-Id: I45913d1fb688c0d91e53333e3450d47e3f2faf62