Implement drag then snap flicker test

Add a flicker test for dragging the pip window
away from the edge and verifying that it snaps
back to that same edge

Bug: 264555500

Test: atest WMShellFlickerTests:PipDragThenSnapTest
Change-Id: I3397f7f47198b33f4d0250704ef5f2fd54446517
(cherry picked from commit d389b970)