Add null check for recents scaling during quick switch.

Test: manual - FallbackRecentsTest.goToOverviewFromHome and
StartLauncherViaGestureTests.testStressPressHome were previously failing
due to null mActivity in AbsSwipeUpHandler#getScaleProgressDueToScroll.
Bug: 192470757
Change-Id: I562171c22ad3c89e64b4dc853a266c8b8b78402c
1 file changed