Removing duplicated code

There's duplicated code in CommonAssertions of WMShell project and FlickerTests. We are keeping the one from FlickerTests and removing the one from WMShell to avoid problems of changing it in one place and forgetiing about the other project.

Test: atest WMShellFlickerTests
Change-Id: Ie87df793933d9cbe0ccd947514027650811a1819