Do not remove/hide surface when waiting for transition start

When the activity is added to DisplayContent#mClosingApps and prepare
for transition, we want to keep the window for the animation.

Bug: 196173550
Test: test with demo app that the activity exit is not blank
Test: atest WmTests:ActivityRecordTests
      #testInClosingAnimation_doNotHideSurface
Change-Id: I4da65a8d302af6ab846a19132397bfbac4ff2f4e