Cancel existing QsClippingAnimation when creating new one

Bug: 191436459

[Manual tests for regressions]
Test: open shade from lockscreen, open fullscreen qs
Test: open unlocked shade, open fullscreen qs
Test: directly open fullscreen qs from top of lockscreen
Test: directly open fullscreen qs via two-finger swipe while unlocked

Change-Id: Ibb8f3e35e5fc14e4e029e98fa3dd83a20b470ebf