Move tests from DisplayPolicyLayoutTests to WindowLayoutTests

This is a step to move the layout logic to the client side. We won't
have DisplayPolicy#layoutWindowLw then.

This CL also renames computeWindowFrames to computeFrames because we
already have "Window" in the class name.

Bug: 161810301
Test: atest DisplayPolicyLayoutTests WindowLayoutTests
Change-Id: I4162e39f0d2aa35141fbbc72e43decd2f3f491f0