Fix EnterSplitScreenNotSupportNonResizable flaky

Before, it called openQuickstep before canSplitScreen, but the later one
is also calling openQuickstep. This caused the recent is pressed twice
to introduce the flakness.

Test: atest WMShellFlickerTests:EnterSplitScreenNotSupportNonResizable
Bug: 182235458
Change-Id: Iba5af5114fd8883b73d78ac34e6f891ee8ca798f