Make sure PipDragThenTest is gravity agnostic

Make sure we check for the correct direction
of the movement as the pip window is getting snapped.

Moreover, make sure waitForPipToSnapTo also checks for
isWMStateComplete in case any other animation is ongoing in WM

Bug: 271864694
Test: atest WMShellFlickerTest:PipDragThenSnapTest
Change-Id: I8626e15d1d368a8a8190f7ceacdf65bdc57486d9
(cherry picked from commit 9ad934f0)
Merged-In: I8626e15d1d368a8a8190f7ceacdf65bdc57486d9