sysui: changed split screen shortcut key behavior.

Instead of cycling between 1/4, 1/2, 3/4 with
consecutive Meta + [ / Meta + ] keys, it dismisses
the activity completely when it goes below 1/4 or 3/4.

Bug: 30075277
Change-Id: I326f4d163537ae96f1845714a8bd56b2b0d58984
(cherry picked from commit b29eeada668b298f9c0e41bd4d4b9fe1f690247e)